

DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director 

Water-Supply Paper 229 


THE DISINFECTION OF SEWAGE AND 
SEWAGE FILTER EFFLUENTS 

WITH 4 CHAPTER ON THE 

PUTRESCIBILITY AND STABILITY OF 
SEWAGE EFFLUENTS 


EARLE BERNARD PHELPS 














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DEPARTMENT OF THE INTERIOR 
UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director 


Water-Supply Paper 229 


THE DISINFECTION OF SEWAGE AND 
SEWAGE FILTER EFFLUENTS 

WITH A CHAPTER OH THE ' 

3 i) <j 

PUTRESCIBILITY AND STABILITY OF 
SEWAGE EFFLUENTS 


BY 

EARLE BERNARD PHELPS 

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WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1909 
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CONTENTS. 



Page. 


Disinfection of sewage and sewage-filter effluents. 7 

Introduction. 7 

Sewage purification. 7 

Purifying agents. 7 

Fate of bacteria in sewage filtration. 8 

Necessity for disinfection. 12 

Methods of disinfection. 15 

Classification of methods. 15 

Heat. 15 

Lime. 16 

Acids. 16 

Ozone. 17 

Chlorine and its compounds. • . 17 

Germicidal action. 17 

Chlorine gas. 17 

Hypochlorites. 18 

Use in disinfection. 18 

Hamburg experiments. 19 

Berlin experiments. 21 

Experiments at Bengal, India. 24 

Experiments in Ohio. 25 

Electrolytic chlorine processes. 25 

Electrolytic manufacture of chlorine. 25 

Webster process. 26 

Woolf process — Electrozone. 27 

Hermite process. 27 

Oxychloride process. 28 

Copper and its compounds. 29 

Miscellaneous methods. 32 

Permanganates. 32 

“Amines” process. 32 

Sodium benzoate and other organic compounds. 33 

Summary of disinfection methods. 33 

Experimental investigations. 34 

History. 34 

Bacteriologic methods and expression of results. 35 

Investigations at Boston. 37 

Scope and character of experiments. 37 

Preliminary experiments with chloride of lime. 38 

Disinfection of trickling-filter effluent. 39 

Disinfection of crude sewage. 46 

Disinfection of septic sewage at Red Bank, N. J. 51 

Disinfection of trickling-filter effluent at Baltimore, Md. 56 

Comparative germicidal efficiencies of chlorine and some of its com¬ 
pounds. 60 

Effect of calcium hypochlorite on colon and typhoid bacilli. 62 


3 
















































4 


CONTENTS. 


Page. 

Disinfection of sewage and sewage-filter effluents—Continued. 

Practical applications and costs of disinfection. 63 

Conclusion. 70 

Putrescibility and stability of sewage-filtep effluents. 74 

Introduction.„. 74 

Putrescibility. 75 

Relative stability. 77 

Definitions. 77 

Estimation of reducing time. 78 

Theoretical relation between reducing time and relative stability.... 80 

Determination. 83 

Incubation periods. 83 

Effect of temperature on reducing time. 86 

Summary of method. 87 

Interpretation of results..".'.. 88 


DISINFECTION TABLES. 


Table 1. Bacteria in sewage and in effluents of sewage filters at Boston in 1904. 11 

2. Bacteria in sewage, septic effluent, and trickling-filter effluents at 

Boston in 1906. 11 

3. Disinfection of three crude hospital sewages with chloride of lime... 20 

4. Disinfection of the crude screened sewage of Eppendorfer, Germany, 

by chloride of lime. 21 

5. Disinfection of crude Berlin sewage with chloride of lime. 22 

6. Destruction by chloride of lime of B. coli embedded in gelatine_ 23 

7. Disinfection of septic sewage by chloride of lime at Bengal, India.. 25 

8. Disinfection of effluents with chloride of lime at Lancaster and 

Marion, Ohio. 25 

9. Analyses of London sewage before and after treatment by the Webster 

process. 26 

10. Summary of Rideal’s experiments on the use of oxychloride at Guil¬ 

ford, England. 28 

11. Disinfection of trickling-filter effluents with copper sulphate at 

Columbus, Ohio. 29 

12. Disinfection of sewage and effluents with copper sulphate at Colum¬ 

bus, Ohio. 30 

13. Effect of organic matter and of alkalinity on the germicidal properties 

of copper sulphate. 31 

14. Disinfection of trickling-filter effluent with copper sulphate at Bos¬ 

ton . 31 

15. Disinfection of effluents with copper sulphate in Ohio. 32 

16. Chemical analysis of trickling-filter effluent at Boston before and after 

disinfection with chloride of lime and sedimentation. 40 

17. Disinfection of trickling-filter effluent with chloride of lime at 

Boston; weekly averages. 42 

18. Disinfection of trickling-filter effluent at Boston; summary of bac¬ 

teriological results averaged by periods to show the effect of 
changes in temperature and in the amount of available chlorine.. 43 

19. Relation between individual tests of bacterial removal and the 

average result at Boston. 44 































DISINFECTION tables. 


5 


Page. 


Table 20. Relation between individual- bacterial counts and the average 

result at Boston. 44 

21. Relation between time of contact and efficiency of disinfection 

with chloride of lime. 45 

22. Disinfection of crude sewage with chloride of lime at Boston; series 

1 and 2. 48 

23. Disinfection of crude sewage with chloride of lime at Boston; series 3. 49 

24. Disinfection of crude sewage with chloride of lime at Boston; sum¬ 

mary of results. 50 

25. Disinfection of septic sewage with chloride of lime at Red Bank, 

N. J.; weekly averages. 54 

26. Relation between individual tests of bacterial removal and the 

average result at Red Bank, N. J... 55 

27. Chemical analyses of trickling-filter effluent at Baltimore, before 

and after disinfection with chloride of lime and sedimentation... 57 

28. Disinfection of trickling-filter effluent with chloride of lime at Balti¬ 

more; weekly averages. 58 

29. Relation between individual tests of bacterial removal and the 

average result at Baltimore... 58 

30. Relation between individual bacterial counts and the average result 

at Baltimore. 59- 

31. Average bacterial removal during the disinfection of trickling-filter 

effluent at Baltimore.. 59 

32. Relative germicidal properties of chlorine and some of its compounds 60 

33. Comparative resistance to calcium hypochlorite of B. typhi and B. 

coli in aqueous emulsion. 63 

34. Estimates of the cost of maintenance and operation of a plant for 

disinfecting sewage or effluent with chloride of lime; based on a 
capacity of 5,000,000 gallons a day. 66 


RELATIVE STABILITY TABLES. 


Table 1. Relation between reducing time and relative stability at 20° centi¬ 
grade . 82 

2. Summary of stability tests of trickling-filter effluents, showing time 

required to exhaust the available oxygen. 84 

3. Comparison of relative stability results obtained from the twenty-day 

incubation period shown in Table 2 with those calculated from a 
four-day period. 85 

4. Comparison of relative stability results obtained from smaller num¬ 

bers of samples by the use of four, six, and twenty day incubation 
periods.,. 85 

5. Relation between relative stability and reducing time at 20° and at 

37° centigrade. 87 


ILLUSTRATION. 


/ 


Plate I. Plan of sewage-disposal works at Red Bank, N. J. 


52 

































THE DISINFECTION OF SEWAGE AND SEWAGE FILTER 

EFFLUENTS. 


By Earle Bernard Phelps. 


INTRODUCTION. 

The investigations on which this report is based were conducted 
by E. B. Phelps at the sanitary research laboratory and sewage 
experiment station of the Massachusetts Institute of Technology 
at Boston, Mass., and, in collaboration with Mr. Phelps, by Francis 
E. Daniels at the sewage-disposal works at Red Bank, N. J., and by 
Ezra B. Whitman at the Walbrook Testing Plant at Baltimore, Md., 
under cooperative agreements with the Massachusetts Institute of 
Technology, the State Sewerage Commission of New Jersey, and the 
city Sewerage Commission of Baltimore. 

SEWAGE PURIFICATION. 

PURIFYING AGENTS. 

The essential agents of sewage purification are provided and em¬ 
ployed by nature. The slow action of the soil bacteria, aided by 
atmospheric oxygen, eventually converts into harmless mineral 
ingredients all organic matter that comes within its sphere of activ¬ 
ity, the process being analogous to that of combustion. Sewage 
purification as practiced to-day is but the intensive application of 
these natural processes under controllable conditions. The improve¬ 
ments that have been made in methods of treating sewage have not 
involved the discovery or application of new principles, but have 
merely increased the working efficiency of the natural bacterial 
agencies. The constant aim of the experimenters has been to in¬ 
crease the rate at which sewage can be treated on a given area of 
land. From the old-time sewage irrigation field, with its maximum 
capacity of possibly 10,000 gallons an acre in twenty-four hours, to 
the present-day trickling filter capable of dealing with two or three 
million gallons an acre a day, the march of improvement has been 
steady and continuous. The amount of sewage that can be purified 

7 




8 DISINFECTION OF SEWAGE. 

on an acre of filtering area has been increased two or three hundred 
fold, and investigators are working toward a still greater increase. 

It must be admitted that the significance of the word “purify” has 
also undergone a radical change. The effluents are no longer pure 
ground water. The liquid flowing from a modern trickling filter 
looks to the untrained eye like the original sewage. The organic 
matter of the sewage is no longer “burned up” to harmless mineral 
matter; indeed, there is almost as much organic matter in the effluent 
as in the raw sewage, and sometimes more. What change then has 
taken place to justify the use of the term “purified?” The answer 
lies in the fact that the organic matter has been changed but not 
removed. To carry out the simile, the organic matter, though not 
burned, has been charred or partly oxidized, and this charring 
process has been sufficient to rob it of its putrescibility or foulness. 
In other words, its chemical composition has been so altered that it 
is no longer capable of undergoing rapid putrefactive decomposition. 

On first consideration it appears inconceivable that the chief 
object of sewage disposal—prevention of the fouling of streams— 
could be attained by such subtle changes in the nature of the organic 
matter. Nevertheless, effluents containing comparatively large 
amounts of organic matter may be discharged into streams with¬ 
out fear of causing nuisances if the organic matter is nonputrescible 
and if conditions preclude immediate sedimentation. The work of 
purification proceeds in the stream as it does in the soil until the 
oxidation, or combustion, is complete, oxygen for that purpose being 
sufficiently abundant in a reasonably clean stream. On the other 
hand, too much crude sewage added to water first robs it of all its 
available oxygen, then, in the anaerobic condition thus established, 
kills the beneficent oxidizing bacteria and transforms the stream 
practically to an open sewer. It is apparent, therefore, that purifi¬ 
cation of sewage has come to mean primarily the removal of its 
tendency to putrefy and not the total oxidation and removal of all 
its organic matter. 

FATE OF BACTERIA IN SEWAGE FILTRATION. 

In the older and more perfect methods of sewage purification the 
bacteria of the sewage with the other organic matter were destroyed 
by the straining action of the soil and the oxidizing action of the 
normal soil bacteria; but a modern filter of coarse stones neither 
strains the material nor affords opportunity for vigorous oxidation. 
Much of the original organic matter passes through such a filter, 
having undergone changes so slight as almost to defy detection by 
ordinary chemical means. It is reasonable, therefore, to inquire as to . 
the fate of the sewage bacteria and particularly as to the pathogens, or 
disease-producing microbes. The data on this point are somewhat 


FATE OF BACTERIA I IT SEWAOE FILTRATION. 


9 


conflicting. The question was raised in 1893 in connection with the 
first septic tank at Exeter, England. The fear was expressed that 
pathogenic germs might even multiply in the tank, as other forms 
of bacteria are known to do. Sims Woodhead conducted an investi¬ 
gation as a result of which he concluded that no organisms capable 
of setting up morbid changes in animals after inoculation came from 
the tank. Pickard introduced an emulsion of typhoid bacilli into 
the Exeter tank and observed a slow diminution in number. It is 
important to note, however, that even after fourteen days 1 per 
cent of the original number was still alive. Pickard also reported 
a removal of over 90 per cent of the typhoid organisms introduced 
into a contact filter. Iiideal, on examining the effluents of three 
Scott-Moncrieff filters at Caterham, England, found reductions in 
Bacillus coli ranging from 95 per cent to 98.5 per cent. He made 
the following statement before the Royal Sewage Commission:® 
“ Satisfactory evidence in most of the systems is now available from 
which I think we are justified in concluding that even if towns on a 
river like the Thames adopted bacterial schemes the pathogenicity 
of the London water supply would not be adversely affected thereby.” 

On the other hand, there is some evidence that the pathogenic prop¬ 
erties of sewage are not materially altered in its passage through a 
coarse-grained filter. Alfred MacConkey b made a series of tests 
upon the longevity of B. typhi in various waters. Samples of the 
liquid under' examination were inoculated with large numbers of 
typhoid bacilli and were kept under observation. In one experi¬ 
ment the organism was isolated from sewage thirteen days after 
inoculation. In a second set of two tests it was not found after fif¬ 
teen and after seventeen days. In two contact-bed effluents it was 
found after fifteen and seventeen days, respectively, while in two 
other contact-bed effluents it did not survive beyond the sixth day. 
MacConkey concluded that the numbers of typhoid organisms reach¬ 
ing a filter are ordinarily so small that there is but slight possibility 
of their passing through, but “if from any cause they arrive to the 
tanks in such large numbers as the B. coli, then certainly they might 
appear in the effluent just as the B. coli does.” In interpreting 
such results due allowance must be made for the fact that the isola¬ 
tion and identification of the typhoid bacillus under such conditions 
is extremely difficult and that failure to detect the organism carries 
much less experimental weight than a positive result. 

Houston, 0 in a careful investigation of the subject for the Royal 
Sewage Commission, found that the effluents from septic tanks, con¬ 
tact beds, and trickling-filter beds contained enormous numbers of 


a Interim Rept. Royal Sewage Commission, 1901, Question No. 4148, p. 251. 
t> Second Rept. Royal Sewage Commission, 1902, p. 62. 
e Houston, A. C., Second Rept. Royal Sewage Commission, 1902, p. 2G.- 



10 


DISINFECTION OF SEWAGE. 


bacteria. In some of the tests the per cent reduction in the effluents 
as compared with the raw sewage was striking; but as it was neces¬ 
sary to judge an effluent by its actual condition, and as the number 
of micro organisms still remaining was almost always very large, he 
concluded that per cent purification is of minor importance. In not 
a few of the tests the bacteria were practically as numerous in the 
effluent as in the raw sewage. The relative abundance of the dif¬ 
ferent kinds of bacteria appeared to be much the same in the efflu¬ 
ents as in the crude sewage. Of undesirable bacteria, such as B. coli, 
proteus-like germs, spores of B. enteritidis sporogenes, and strepto¬ 
cocci, the effluents contained nearly as many as the crude sewage. 
The reduction in numbers of these objectionable bacteria was appar¬ 
ently not marked enough to be of consequence from the point of 
view of the epidemiologist. No definite proof was found that the 
effluents from bacterial beds were conspicuously safer than crude 
sewage in their possible relation to disease. Attention was especially 
called to the presence of streptocbcci in the effluent. Houston con¬ 
tends that if it be true that streptococci are more delicate germs than 
the typhoid bacillus, their presence in large numbers in the effluent 
indicates the possibility or probability that the typhoid bacillus also 
survives under similar conditions — a view that leads to the inference 
that the biological processes at work are not strongly inimical, if 
hostile at all, to the vitality of pathogenic germs. Experiments 
with B. sporogenes, a spore-forming organism, have shown that it 
passes through filters in almost undiminished numbers. 

Besides the facts already mentioned, little information is avail¬ 
able which bears directly on the fate of the pathogens, and particu¬ 
larly of the typhoid organism, in sewage purification. It is there¬ 
fore necessary to examine the available indirect evidence. By study¬ 
ing the removal of certain specific organisms that are easily detected 
and certain well-defined groups of organisms existing in sewage, the 
probability of the elimination of the typhoid organism can be deter¬ 
mined. In the absence of further data it must be assumed for the 
sake of safety that the elimination of the typhoid bacillus is not 
materially greater than that of the other species and groups that 
maybe studied. During the summer of 1904 there were operated at 
the sewage experiment station four septic tanks running at storage 
periods of from twelve to forty-eight hours, nine contact beds differ¬ 
ing in material, depth, and rate, three trickling filters, and three 
sand filters, one of each set being run with crude sewage and two 
kinds of septic effluents, respectively. The results of bacterial counts 
are shown in Table 1, in which the numbers are averages of all results 
obtained with one kind of filter.® 

a Winslow, C.-E. A., The number of bacteria in sewage and sewage effluents: Jour. Infect. Dis., vol. 1, 
Suppl. 1, 1905, p. 209. 





FATE OF BACTERIA IN SEWAGE FILTRATION. 


11 


Table 1 . —Bacteria in sewage and in effluents of sewage filters at Boston in 1904- 

[Winslow.] 


Source of sample. 

Number 
of exam¬ 
inations. 

Bacteria per cubic centimeter. 

Lactose gelatin at 20° C. 

Lactose agar at 37° C. 

Anaerobic 

agar. 

Liquefiers. 

Acid 

formers. 

Total. 

Acid 

formers. 

Total. 

Sewage. 

56 

365,000 

1,670,000 

5,430,000 

1,670,000 

3,760,000 

2,440,000 

Septic effluent. 

56 

162,000 

495,000 

1,750,000 

650,000 

1,040,000 

930,000 

Contact filters. 

140 

60,000 

270,000 

1,060,000 

290,000 

570,000 

440,000 

Trickling filters. 

18 

134,000 

114,000 

451,000 

284,000 

1,170,000 

200,000 

Sand filters. 

15 

500 

1,360 

9,160 

11,400 

43,600 

1,200 


The septic tank and two trickling filters in operation at the same 
station during the summer of 1906 gave the average results recorded 
in Table 2, showing that less than one-half of the bacteria growing at 
37° C. were removed by filtration, and that the reduction in the num¬ 
ber of colon bacilli was practically in the same proportion. 

Table 2. —Bacteria in sewage, septic effluent, and trickling-filter effluents at Boston in 

1906. 


Source of sample. 

Bacteria per 
cubic centi¬ 
meter; lac¬ 
tose agar at 
37° C. 

B. coli; posi¬ 
tive a tests in 
one-millionth 
of a cubic cen¬ 
timeter. 


1,300,000 
750,000 
1,650,000 
750,000 

Per cent. 

65 
35 

66 
35 






a The bile broth presumptive test recommended by D. D. Jackson was employed. 


Johnson,® in his experiments at Columbus, found a reduction in the 
total number of bacteria ranging from 33 per cent to 60 per cent in 
primary-contact filters, and a removal of about 39 per cent of the 
remainder in secondary-contact filters, and from 30 per cent to 80 per 
cent in trickling filters, depending largely on the depth of the filter. 
Subsequent sedimentation increased this removal to 87 per cent. 
Thumm and Pritzkow, 6 at Berlin, report a reduction in the number of 
bacteria from 17,000,000 in the sewage to 6,000,000 in the effluent 
of a double-contact filter. At La Madelein, France, Calmette 6 found 
5,000,000 bacteria per cubic centimeter in crude sewage, 2,900,000 in a 
secondary-contact effluent, and 800,000 in the effluent from a trickling 
filter. The sewage contained 20,000, the contact effluent 4,000, and 
the trickling effluent 2,000 colon bacilli. At Plainfield, N. J., the 

a Johnson, Geo. A., Report on sewage purification at Columbus, Ohio, 1905. 

b Thumm, K., and Pritzkow, A., Mitteilungen aus der Koniglichen Prufungsanstalt fur Wasserversor- 
gung und Abwasserbeseitigung zu Berlin, 1903, vol. 2, p. 127. 

c Calmette, A., Recherches sur l’epuvation des eaux d'egout, Lille, 1907, vol. 2. 







































12 


DISINFECTION" OF SEWAGE. 


double-contact, filter was found to reduce the number of bacteria 
from an average of 1,000,000 per cubic centimeter to an average of 
322,000; B. coli ranged from 1,000,000 to 100,000 in the sewage and 
from 100,000 to 10,000 in the effluent.® An experiment performed by 
Houston 6 is important in this connection: B. pyocyaneus, a patho¬ 
genic organism, was applied to the top of a trickling filter, and ten 
minutes later the bacillus appeared in the effluent, continuing to be 
discharged for ten days. In a similar manner the same organism was 
found to pass through a septic tank and a contact filter successively, 
and to persist in both for nine days. At Baltimore, Md., the board of 
advisory engineers concluded that 95 per cent of the bacteria in the 
sewage could be removed by a system comprising a septic tank, 9-foot 
trickling filters, and supplementary sedimentation basins. 0 Such 
results are better than those usually obtained elsewhere, and if they 
can be maintained in practice they will go far toward solving the 
problem in that locality. 

From a consideration of the available evidence it may be stated in 
a general way that coarse-grained, rapid sewage filters remove a con¬ 
siderable proportion of the sewage bacteria; that such removal has 
not been found to be sufficiently complete in practice to have great 
sanitary significance; that bacteria of various groups and certain 
specific organisms pass through such filters in practically the same 
proportions as the bacteria as a whole; and that, in the absence of any 
information to the contrary, it should be assumed that such filters 
have no greater effect on the typhoid and other pathogenic organisms 
than on B. coli, B. pyocyaneus, sewage streptococci, or the different 
groups of sewage bacteria. 

THE NECESSITY FOR DISINFECTION. 

It is probable that removal of bacteria will again be considered an 
essential factor in sewage purification. That it was so considered 
formerly is well known. The development of the modern rapid filter 
has made it possible to introduce sewage purification under conditions 
where it would have been impossible or prohibitively expensive in 
former days. In the acceptance of a partial solution of the problem 
much has been gained and but little lost. The process of purification 
that renders practically stable the offensive organic matter of sewage 
has accomplished the most important and in many cases the only 
essential requirement. If, however, sewage effluents find their way 
into the drinking waters of neighboring communities, the question of 
the relative responsibility of the settlements is debatable. It is gen¬ 
erally conceded at present that efficient sewage purification should be 

a Rept. Sewerage Commission of New Jersey for 1906. 

b Houston, A. C.', Fourth Interim Rept. Royal Sewage Commission, 1904, vol. 3, p. 77. 

c Report of the Board of Advisory Engineers to the Sewerage Commission of Baltimore, 1906. 



NECESSITY FOR DISINFECTION. 


13 


undertaken by the communities in which the sewage originates, and 
that purification of the water supply is the urgent duty of the other 
communities. As streams flowing through populous districts are 
necessarily contaminated and unfit for domestic use without filtration, 
it is considered unjust to require a community to purify its sewage to a 
higher degree bacterially than that shown by the stream into which it 
is discharged. On the other hand, in the fig ht against infectious dis¬ 
eases, sound tactics demand an attack on the enemy as near as pos¬ 
sible to the initial source of infection. The best and easiest place to 
destroy typhoid germs is at the bedside of the typhoid patient; but 
this method can not be relied on to keep sewage free from infection, 
and the next strategic point is certainly the sewage. Once at large 
the germs -may reach their victims in a score of well-known ways, 
and who will say by how many devious and unknown paths ? With 
increasing knowledge of these facts and with improved processes and 
reduced cost of disinfection it is not too much to expect that the 
disinfection of sewage will some day be regarded as an integral part 
of its purification and as a necessary measure of protection for the 
community. 

One of the ways by which typhoid germs pass from the sewer to 
the victim is by means of shellfish fed or fattened in polluted water, 
and many of the oyster and clam beds of the eastern seaboard are 
subject to pollution by sewage. In England conditions are so 
serious that the demand for shellfish has perceptibly decreased. 
The situation presents obvious difficulties. The water of a polluted 
river may be rendered potable through filtration, but the purification 
of shellfish seems to be out of the question; it follows, therefore, that 
health must be safeguarded either by preventing the discharge of the 
bacteria of sewage on shellfish beds or by prohibiting the taking of 
the shellfish. In a large community where the shellfish industry 
is small it will probably be more satisfactory to adopt the latter 
method, but in small communities whose sewage pollutes large and 
important beds thorough bacterial purification is not an unreason¬ 
able requirement. Unfortunately the greatest shellfish areas of the 
country are situated near large cities, where they are subject directly 
or indirectly to possible pollution from sewage discharge. 

In a carefully prepared paper on the pollution of shellfish beds, 
G. W. Fuller® states that the annual crop of oysters gathered along 
the Atlantic and Gulf coasts in 1902 amounted to more than 
25,000,000 bushels, exceeding in value $13,000,000, and that the 
crop of clams was more than 2,000,000 bushels, valued at $2,000,000. 
Over one-half of this total production came from New Jersey, 
Maryland, and Virginia, and the shellfish were grown mainly in the 


a Concerning sewage disposal from the standpoint of the pollution of oysters and other shellfish, with 
especial reference to their transmission of typhoid fever: Jour. Franklin Inst., vol, 160,1905, p. 82. 



14 


DISINFECTION OF SEWAGE. 


waters of Delaware and Chesapeake bays, which receive the sewage 
from many large cities. At present this sewage is so enormously 
diluted by the bay waters that the danger of pollution is in most 
places remote, but it is present and ever increasing. 

In Baltimore a system of sewerage and sewage disposal is being 
planned to remedy the already serious pollution of Patapsco River. 
In calling on a commission of experts for advice the sewerage com¬ 
mission of Baltimore specified “that the effluent proposed to be 
discharged into Chesapeake Bay or its tributaries in the system to 
be recommended by the engineers shall be of the highest practicable 
degree of purity.” ° 

By agreement between the States of New Jersey and Penn¬ 
sylvania the condition of Delaware River has been thoroughly 
investigated, and the pollution of that stream by sewage from 
Trenton, Bordentown, and other smaller communities in New Jersey, 
and Philadelphia, Easton, and other Pennsylvania cities, is pro¬ 
hibited after certain specified dates. 6 The necessity for protecting 
these valuable she llfi sh beds makes bacterial removal an essential 
feature in any scheme of sewage disposal which may be considered 
for such places. 

The board of advisory engineers at Baltimore recommended that 
the settled trickling-fflter effluent be given a final treatment on sand 
filters. The cost of works for the complete treatment of 75,000,000 
gallons of sewage a day by septic tanks, trickling filters, sedimen¬ 
tation basins, and sand filters was estimated at $3,283,250, of 
which sum $1,040,750, or over 31 per cent, was for supplementary 
treatment. The annual cost of operation is-expected to be $115,500, 
of which $55,000, or 48 per cent, is for supplementary treatment. 
This gives some idea of the cost of complete bacterial removal by 
filtration processes over and above the cost of reasonable organic 
purification. In regard to disinfection by chemical means the 
advisory engineers state: “To remove all bacteria remaining in the 
settled effluent from the sprinkling filters by disinfectants, such as 
hypochlorite of lime or of sodium or sulphate of copper, would be 
prohibitively expensive”-—an authoritative opinion based on the 
best evidence then to be had. Almost no American data on chemical 
disinfection were available, and the results of experiments in 
Germany indicated that such disinfection could be accomplished only 
at high cost. The patented processes mainly used in England were 
also expensive. It was therefore desirable to learn just how effective 
disinfection processes could be made under conditions in America— 
how much they would cost, and what after effects, objectionable or 
otherwise, might follow their introduction. 


a Report of the Board of Advisory Engineers to the Sewerage Commission of Baltimore, 1906. 
b Rept. New Jersey State Sewerage Commission, 1907. 



METHODS OF DISINFECTION. 


15 


A review of the available data and a short experimental investiga¬ 
tion made in 1906 at Boston® led the writer to believe that there is 
much value in the process and that it might afford the best possible 
solution of the whole problem under certain conditions common in 
this country, particularly in localities where the shellfish question is 
involved. 

METHODS OF DISINFECTION. 

CLASSIFICATION OF METHODS. 

In Great Britain the somewhat indefinite allusions of the royal 
sewage commission to sterilization as a finishing process in sewage 
treatment aroused a storm of discussion that resulted., at least, in 
clearing away many misconceptions. Sterilization processes of many 
kinds have been investigated, and the subject has been freely dis¬ 
cussed. For the following classification of sterilizing agents and for 
many of. the facts noted here the writer is indebted to Rideal, 6 whose 
carefully prepared paper on the subject discusses its possibilities in a 
thoroughly impartial manner. Other authorities are cited wherever 
possible. Except when otherwise stated, costs are based on current 
prices in eastern markets. 

The different methods and substances proposed for the sterilization 
of effluents are considered in the following order: 

1. Heat. 

2. Lime. 

3. Acids. 

4. Ozone. 

5. Chlorine and its compounds. 

(а) Chlorine gas. 

(б) Hypochlorites, or oxychlorides. 

(c) Electrolytic chlorine processes. 

6. Copper and its compounds. 

7. Miscellaneous. 

(а) Permanganates. 

(б) “Amines” process. 

(c) Sodium benzoate and other organic compounds. 

HEAT. 

The use of heat has been suggested for disinfecting sewage. In his 
testimony before the royal sewage commission E. E. Klein 0 referred 
to a patented process which he considered practicable, by the use of 
which sufficient ammonia could be recovered from the sewage nearly 
to pay for the treatment; but there is no record that this process 

a phelps, Earle B., and Carpenter, William T., The sterilization of sewage-filter effluents: Tech. Quart., 
vol. 19,1906, p. 382; Contributions from the Sanitary Research Laboratory, vol. 4, 1908. 

b Rideal, S., On the sterilization of effluents: Jour. Royal Sanitary Institute, vol. 26, p. 378. 

c Interim Rept. Royal Sewage Commission, 1901, question 9674, p. 619. 

76474— irr 229—09-2 




16 


DISINFECTION OF SEWAGE. 


has ever been used on a working scale. A device for heat interchange 
similar to that employed in the Forbes sterilizer may possibly, as is 
claimed, raise water to the boiling point and cool it again within 5° F. 
of the initial temperature. With coal at $3 per ton having a calorific 
value of 10,000 British thermal units, the fuel cost alone of such an 
operation would be about $7 per million gallons of sewage. The 
ammonia in a million gallons of Boston sewage, if in the form of 
sulphate, would have a market value of $20. Whether the difference 
between the value of the ammonium sulphate and the cost of the 
fuel is sufficient to cover the cost of operation, including labor, 
evaporation of the dilute solution, and all fixed charges can be deter¬ 
mined only by actual experiment, but the plan is not wholly without 
possibilities. 

LIME. 

Caustic lime acts only slightly as a germicide, and the considerable 
removal of bacteria that takes place when lime is used as a precipitant 
in sewage is undoubtedly due to the action of the precipitate itself 
in dragging down with it the bacteria which it has entangled. Such 
action occurs to some extent in the precipitation of any substance, 
and even the sedimentation of sewage is always accompanied by 
reduction in the numbers of bacteria. Lime alone, therefore, would 
be of little value for sterilizing effluents. Rideal states that 60 to 70 
grains per imperial gallon (860 to 1,000 parts per million) are inefficient 
in sterilization. Thresh, however, believes that lime would produce 
a satisfactory sterilization of effluents.® 

ACIDS. 

Most bacteria, and particularly typhoid and cholera germs, are 
more readily destroyed by acids than by alkalies. Rideal, therefore, 
considers it feasible to employ acids as germicides. He states that 
Stutzer found 0.05 per cent acid solutions fatal to bacteria in twenty- 
four hours; that Ivanoff found that 0.04 per cent to 0.08 per cent of acid 
destroyed the cholera germs in the sewage of Berlin and of Potsdam; 
that Kitasato found 0.08 per cent of sulphuric acid fatal to typhoid 
bacilli in fifteen minutes, and that he had himself obtained similar 
results. To furnish 1,000,000 gallons of sewage with 0.08 per cent of 
sulphuric acid requires 6,650 pounds of acid, costing approximately 
$73. Smaller amounts of acid might be used, as it would be unnec¬ 
essary to kill typhoid germs in so short a time as fifteen minutes; but, 
on the other hand, most sewage contains a considerable amount of 
free alkali which must be neutralized before any germicidal effect of 


a Interim Rept. Royal Sewage Commission, 1901, question 8917, p. 602. 



METHODS OP DISINFECTION. 


17 


the acid would be obtained. This process consequently would seem to 
be impracticable except in emergencies. It is interesting to note that 
the sewage of Worcester, Mass., contains normally an average of 0.01 
per cent of free sulphuric acid, or half enough to kill cholera germs in 
twenty-four hours. 

OZONE. 

Ozone has been used more or less successfully in Germany, particu¬ 
larly at Weisbaden, for sterilizing drinking water. Though the proc¬ 
ess has been most favorably commented on by those in immediate 
charge of the investigations, it has not been generally regarded as 
successful. The possibility of procuring a satisfactory effluent by 
this process, when the water is that of a highly-polluted river is be¬ 
yond question, but whether the process can satisfactorily treat a sew¬ 
age effluent of considerable turbidity has not been determined. 
Rideal calls attention to the fact that ozone is but sparingly soluble 
in water, and on that account it might fail to penetrate the solid 
masses in the effluent, since the rate at, which a dissolved gas will 
penetrate solids in a liquid is a direct function of its solubility. The 
principal cause of failure of the ozone process, however, seems to be 
its expense. If this be true in waterworks it is hardly possible that 
such treatment could be applied to sewage effluents as an additional 
safeguard after purification. Data are not at hand for estimating 
the cost of applying ozone treatment to sewage effluents, but besides 
the cost of operation there is the very considerable cost of installing 
the necessary machinery and towers. 

CHLORINE AND ITS COMPOUNDS. 

GERMICIDAL ACTION. 

Chlorine is well known as a powerful germicide. As a bleaching 
agent it acts on organic coloring matter indirectly by means of the 
free nascent oxygen which it liberates from the water in which it is 
dissolved, and it is probable that its germicidal action is similar. In 
other words, chlorine and ozone owe their germicidal power to the 
same thing—nascent oxygen. Chlorine, however, has the advan¬ 
tages of being cheap, of being more readily soluble, and of being 
obtainable in compounds that are easily transported and handled. 

CHLORINE GAS. 

Until within a few years chlorine has been manufactured commer¬ 
cially by the Weldon or some similar process. In the Weldon process 
hydrochloric acid is made to react with a complex mixture of man- 


18 


DISINFECTION OF SEWAGE. 


ganese hydroxide and lime — “Weldon mud” — the reaction being 
essentially 

Mn0 2 + 4HC1 = MnCl 2 + 2H 2 0 + Cl 2 , 

though in reality it is much more complex. Recently, however, 
electrolytic processes have been developed, by which the cost of 
manufacture has been materially reduced, particularly where cheap 
water power is available. Chlorine prepared by the mixing of com¬ 
mon salt, an acid, and a suitable oxidizing agent was used in England 
as early as 1800 by Cruikshank, who recommended manganese 
dioxide and potassium bichromate as oxidizing agents. The objec¬ 
tions to the use of gaseous chlorine are chiefly the cost of transporta¬ 
tion, the difficulty and danger of handling the gas, and the difficulty 
of measuring accurately the amount of gas added to the effluent. 
Furthermore, in disinfection free chlorine is not so efficient as the 
hypochlorite. 

Available chlorine, a term frequently used in the discussion of 
chlorine disinfection methods, is determined by titrating a solution 
with arsenious acid, or with some other reducing agent, and it 
represents in reality the oxidizing power of the substance expressed 
in terms of chlorine. For example, hypochlorous acid in the pres¬ 
ence of a reducing agent is decomposed according to the following 
equation: 

HC10 = HCl + 0. 

The oxidizing power of this acid, or the available chlorine, is, there¬ 
fore, two hydrogen equivalents per molecule, which is twice its total 
chlorine content, a fact that makes the term available chlorine a 
misnomer, but it has come into general use in the chlorine industries 
and it is a convenient expression. In this article it signifies oxidizing 
power, determined against arsenious acid, and expressed in terms of 
chlorine. 

HYPOCHLORITES. 

Use in disinfection .—Chlorine is handled commercially in the form 
of bleaching powder, or chloride of lime — an impure product com¬ 
posed largely of calcium hypochlorite. Bleaching powder or “bleach” 
containing from 35 per cent to 40 per cent of available chlorine can 
be obtained in the market. The hypochlorite dissolves in water, 
leaving a residue composed chiefly of calcium hydrate and calcium 
carbonate. Hypochlorites in general are made by adding chlorine 
to caustic alkalies. Bleaching powder is made by passing dry chlo¬ 
rine gas over freshly-slaked lime. It is manufactured in this country 
in large amounts, the chlorine being obtained by the electrolysis of 
salt. Abroad the chlorine is made by the older methods, and much 


METHODS OP DISINFECTION. 


19 


foreign bleach is sold in this country in competition with the electro¬ 
lytic product. “Eau de Javelle” and “Labarraque’s solution” are 
solutions of sodium hypochlorite. “Chloros,” a commercial prepara¬ 
tion of sodium hypochlorite, contains 10 per cent by weight of avail¬ 
able chlorine. 

Hypochlorites have long been recognized as powerful and efficient 
disinfectants. The sodium and potassium compounds have not been 
generally used on a large scale because of their relatively high cost 
and the difficulty of preparing and keeping them in the dry state, 
but calcium hypochlorite has been extensively employed. The first 
Royal Sewage Commission of Great Britain used it in deodorizing 
London sewage in 1854.“ The committee of 1885 of the American 
Public Health Association found it to be the best disinfectant availa¬ 
ble, cost and efficiency considered. It was used by Dibdin * 6 in 1884 
to deodorize the sewage of London, but it was not successful for that 
purpose and was later abandoned in favor of sodium permanganate. 
Its action on specific bacteria was studied by Nissen c in 1890. The 
use of bleaching powder as a sewage disinfectant has been more 
extensively studied in Germany than elsewhere. At the Hygienic 
Institute of Hamburg investigations have been made by Proskauer 
and Eisner, Dunbar and Zirn, Dunbar and Korn, Schumacher, and 
Schwarz. At the Royal Testing Station in Berlin, Kranepuhl and 
O. Kurpjuweit have each reported investigations. 

Hamburg experiments .—Proskauer and Eisner d experimented at 
Hamburg with sewage which had been clarified by the Rothe-Degener 
system. They obtained satisfactory disinfection with chloride of 
lime, using concentrations of chlorine ranging from 2.7 to 4.0 parts 
per million, and ten minutes exposure sufficed practically to eliminate 
B. coli. Dunbar and Zirn d treated crude sewage and, in common 
with later workers, they imposed much more exacting standards of 
disinfection and used much greater concentrations of chlorine. 
After having employed cholera germs as test organisms, they con¬ 
cluded that the satisfactory disinfection of crude Hamburg sewage 
would require 25 parts per million of available chlorine and an exposure 
of two hours. Dunbar and Korn e studied the disinfection of crude 
sewage with special reference to its subsequent purification on bio¬ 
logical filters. Schumacher^ investigated the problem of disinfecting 
hospital sewages that had not received any previous treatment. In a 

a Second Kept. Royal Sewage Commission, London, 1861. 

6 Dibdin, W. J., Jour. Assoc. Eng. Soc., vol. 40, 1908, p. 310. 
c Zeit. Hyg., vol. 8, 1890, p. 62. 

d Vierteljahrsschr. ger. Med., vol. 16,1898, Suppl. Heft. * 

« Ges. Ing., vol. 27, 1904. 
f Idem, 1905. 



20 


DISINFECTION OF SEWAGE. 


preliminary set of bottle experiments he obtained the following 
results with the sewages of three hospitals. 

Table 3. —Disinfection of three crude hospital sewages with chloride of lime. 


[Schumacher.] 


Concentration of chloride of lime. 

Hours of 
contact. 

1 Bacteria 

Initial con¬ 
tent: 

| 23,000,000. 

per cubic ce 

Initial con¬ 
tent: 

37,000,000. 

ntimeter. 

Initial con¬ 
tent: 

21,000,000. 

After treat¬ 
ment. 

After treat¬ 
ment. 

After treat¬ 
ment. 


f ' 2 

540 

200 

8,400 

1:7,000. 

4 

140 

260 

100 


6 

200 

100 

20 

. 

{ 24 

160 

20 

60 


f 2 

60 

60 

100 

1:5,000. 


160 

40 

800 


6 

120 


600 


24 

30 





80 

420 

40 

1:2,000. 

i 4 

20 


60 


1 6 

20 

20 

20 


f 2 

60 

20 

80 

1:1,000... 

4 

120 

40 

180 


6 

40 

40 



The amount of available chlorine in the chloride of lime used in 
these experiments is not stated, but it was probably not far from 30 
per cent. On that assumption the concentration of available chlorine 
in the four sets of tests would be 43, 60, 150, and 300 parts per million, 
respectively. The high initial numbers indicate a very strong sewage. 
It is also worthy of note that the reduction in the number of bacteria 
with only 43 parts of chlorine is much greater than would be demanded 
in ordinary practice. Disinfection on a large scale was also conducted 
by the same investigator at two hospitals. After a storage period of 
two hours samples of one liter each were tested for B. coli. With a 
concentration of 1: 2,000, or about 150 parts per million of available 
chlorine, B. coli was isolated from a liter of water in only 6 samples 
out of 43, not being found in 88 per cent of the samples tested. With 
chloride of lime in the proportion of 1: 5,000, or about 60 parts per 
million of available chlorine, B. coli was destroyed in 62 per cent of the 
samples in two hours and in 64 per cent of the samples in four hours. 
Schwarz a called attention to the fact that excessive amounts of 
disinfectant are necessary on account of the large floating particles, a 
point previously commented on by Schumacher. Schwarz proposed, 
therefore, that all sewage should be carefully screened before disin¬ 
fection in order to remove particles exceeding three millimeters in 
diameter. The screening of hospital sewage should be even more 
complete, removing particles exceeding one millimeter in diameter. 


Ges. Ing., vol.. 29, 1906, p. 773. 
























METHODS OE DISINFECTION. 


21 


During experiments at the Eppendorfer purification works, a sewage 
flow of about 60,000 United States gallons a day was available, and a 
tank holding about four hours’ flow was used. As information was 
especially desired regarding the effect of the treatment on the cholera 
vibrio, an emulsion of another vibrio (Leuchtvibrionen) was added to 
the sewage at a regular rate before treatment, after it had been deter¬ 
mined that this test organism would not only pass through the tank 
but would persist for days in the filters. Table 4 summarizes the 
results. 


Table 4. — Disinfection of the crude screened sewage of Eppendorfer, Germany, with chlo¬ 
ride of lime. 

[Schwarz.] 



Tests for vibrio in 1 cu¬ 
bic centimeter amounts. 

Test for B. coli in 1 cu¬ 
bic centimeter amounts. 

Final num¬ 
ber of bac¬ 

Concentration of chloride of lime.o 

Total num¬ 
ber of tests. 

Number of 
positive 
tests. 

Total num¬ 
ber of tests. 

Number of 
positive 
tests. 

teria per 
cubic centi¬ 
meter.!) 

1:2,000.. 



17 

0 

15 

1:5,000. 

51 

0 

51 

1 

23 

1:10,000. 

28 

0 

7 

0 

36 

1:20,000.. 

15 

0 

6 

0 

72 

1:30,000. 

10 

0 


3,620 
59,000 
950,000 

1:40,000. 

8 

4 



Control 











a 1:10,000 is about 30 parts per million of available chlorine, assuming that the chloride of lime contained 
30 per cent of available chlorine. 
b Initial number of bacteria per cubic centimeter, 1,350,000. 


It was noted that a concentration of chloride of lime of 1: 2,000 
materially affected the subsequent treatment of the disinfected 
sewage in trickling filters. Oxygen consumed and ammonia in the 
effluent were higher and nitrates lower than normal. An interesting 
fact noted was the production of chlorates in the filter, over 10 parts 
per million being recorded at one time. Schwarz concluded that 
sewage can be satisfactorily disinfected with chloride of lime after 
having been carefully passed through one millimeter mesh screens. 
One part in 5,000 (60 parts per million of available chlorine) was 
considered necessary for the destruction of typhoid germs and from 
one part in 7,000 to one part in 10,000 (30 to 40 parts of available 
chlorine) for cholera vibrio. The, disinfected sewage can be sub¬ 
sequently purified without previous neutralization of the disinfectant. 

Berlin experiments . — At the royal testing station in Berlin the 
subject of sewage disinfection has been studied by Kranepuhl* and 
by Kurpjuweit. & Kranepuhl undertook to determine the concentra¬ 
tion of chloride of lime and the time of contact necessary to destroy 
the colon bacilli in crude Berlin sewage. These bacilli numbered 

a Mitteilungen aus der Konighlichen Prufungsanstalt fur Wasserversorgung und Abwasserbeseitigung zu 
Berlin, vol. 9,1907, p. 149. 
b Idem, p. 162. 
























22 


DISINFECTION OF SEWAGE. 


about 100,000 per cubic centimeter. They were considered an 
index of the pathogenicity of the sewage, because they are more 
numerous and more resistant than the pathogenic forms. The 
bleaching powder employed contained available chlorine ranging 
from 25 to 35 per cent. One liter samples of sewage were treated 
with the desired amounts of chloride of lime, and at the expiration 
of the specified time the remaining available chlorine was deter¬ 
mined and was then neutralized with sterile sodium thiosulphate. 
Nutrient broth was then added to the entire liter sample, after which 
the sample was incubated. B. coli was sought in the incubated 
sample by the usual means, and confirmatory tests for it were made. 

Kranepuhl’s results are summarized in Table 5, in which positive 
tests mean that B. coli was found in 1 liter. 


Table 5. —Disinfection of crude Berlin sewage with chloride of lime. 
[RranepuhL] 


Available 


' B. coli in liter samples. 

chlorine (in 
parts per 
million.) 

Time of 
exposure. 

Number of 
samples 
tested. 

Number of 
positive 
tests. 

Per cent of 
positive 
tests. 

50 

Hours. 

2 

20 

11 

55 

50 

4 

9 

2 

22 

60 

2 

17 

6 

35 

60 

4 

6 

3 

50 

150 

2 

19 

4 

21 

150 

4 

10 

1 

10 

300 

2 

16 

1 

6 

300 

4 

7 

0 

0 


Kurpjuweit® studied the penetration of solid particles by the dis¬ 
infectant. He made test cubes of gelatine having a volume of about 
10 cubic centimeters, which he immersed in solutions of chloride of 
lime for definite times, then removed and melted in warm water, 
after which he determined the available chlorine. He found the 
amount of available chlorine to be a regular function of the time of 
exposure and of the concentration of the solution. The most striking 
fact noted was the small quantity of chlorine actually absorbed. 
For instance, a cube that had been immersed for ninety hours gave 
a mean concentration of chlorine within its own volume equal to 
but 1 per cent of the concentration of the solution. There was also 
shown to be a chemical combination between the chlorine and the 
gelatine. In order to determine whether such chlorine was active 
in disinfection before it became combined, similar gelatine cubes, 
inoculated with B. coli before setting, were immersed for two hours 
in solutions containing from 60 to 3,000 parts per million of available 
chlorine. The results in Table 6 were obtained. 


Loc. cit. 












METHODS OP DISINFECTION. 


23 


Table 6.— Destruction by chloride of lime of B. coli embedded in gelatine. 
[Kurpjuweit.] 


Concentration of solution (average available chlorine in parts per million). 


Number of 
B. coli re¬ 
maining in 10 
cubic centi¬ 
meters after 
2 hours. 


0 (control). 

60. 

150. 

300. 

300. 


91.500 
55,400 

43.500 
48,100 

6,600 


The value of these results in practical work is problematic for several 
reasons. The cubes employed are much larger than the particles that 
should be in any sewage to be disinfected, and the character of the 
material is still more significant, because cubes of solid gelatine do 
not represent in any way the porous, semisoluble masses that occur 
in sewage. The question of penetration is an important one, and 
there can be no doubt that the practical efficiency of disinfection 
processes is limited by the ability of the disinfectant to penetrate 
small, solid particles that may be in the sewage. This point has 
been illustrated in a practical manner by Kurpjuweit’s experiments. 
Four samples of crude sewage were screened through sieves having 
openings 2, 5, 7, and 10 millimeters in diameter, respectively. Four 
portions of each of the filtrates thus obtained were then treated with 
chloride of lime, so proportioned that the available chlorine was 150, 
300, 600, and 3,000 parts per million, respectively. In the sewage 
that was screened through a 2-millimeter mesh, 150 parts per million 
of available chlorine destroyed all the B. coli in four separate liter 
samples, while the same concentration of chlorine destroyed the B. 
coli in only 5 out of 8 liter samples screened through the 10-millimeter 
mesh. Indeed, 3,000 parts of available chlorine were required to 
remove completely the B. coli in the samples screened through the 
10-millimeter mesh. 

It should be noted that the investigators at Hamburg and at Berlin 
dealt wholly with crude sewage. Even when purification plants are 
in operation the disinfection is invariably applied first. No good 
reason for this procedure is obvious, unless it is that preliminary sedi¬ 
mentation tanks are available and supplementary tanks are not. In 
Germany the method of operation is possibly justified by the fact 
that the processes are being studied in order that they may be adopted 
for temporary use during epidemics, and it is not proposed to practice 
disinfection regularly. The expense of treating crude sewage, how¬ 
ever, is at least twice that of treating a well-purified filter effluent. 
The very high standards that have been established for this work are 














24 


disinfection of sewage. 


also of interest. It is proposed so to treat a sewage containing over 
100,000 B. coli per cubic centimeter that the number of that kind of 
bacillus will be reduced to less than one in a liter. The result can 
hardly be expressed in per cent purification, and it is proper to inquire 
why such severe standards are employed. If the number of colon 
bacilli were reduced even to one per cubic centimeter, it would mean 
a reduction of 99.999 per cent. It may safely be inferred that the 
number of typhoid and cholera germs would be reduced in about the 
same ratio, and furthermore that the number of typhoid and cholera 
cases due to the discharge of this sewage would be similarly decreased. 
In other words, if such disinfection were generally adopted, 99,999 
cases of disease out of every 100,000, which are due, directly or indi¬ 
rectly, to sewage pollution, would be eliminated. Such reduction 
would seem to be very satisfactory, and yet it is proposed to improve 
this a thousand fold by insisting on an elimination of the colon 
bacilli from 1-liter samples, thus making the cost so great that it 
practically prohibits the use of the process, except for short periods 
during serious epidemics. It is well worth considering whether a con¬ 
tinuous removal of 99 per cent of the disease germs is not a better 
safeguard of the public health than an occasional complete removal 
during epidemics. 

Experiments at Bengal , India .— The government of Bengal,® in 1904, 
appointed a commission to report on the pollution of Hooghlv River 
by the effluents of septic tanks. Though the commission decided 
that the physical and chemical pollution of the river by the effluents 
was improbable, as sufficient dilution took place at all seasons to pre¬ 
vent any nuisance, bacterial purification of the effluents was deemed 
advisable. Experimental sand filters and copper sulphate disinfec¬ 
tion satisfactorily removed the germs, but substitution of chloride 
of lime for the copper salt accomplished the same result at much less 
expense. 

A septic tank installed near Calcutta was connected with a 
public latrine serving about 2,000 persons. From 400 gallons to 
2,500 gallons of septic-tank effluent were daily treated with various 
amounts of chloride of lime, the available chlorine in which ranged 
from 20 to 60 parts per million. The numbers of bacteria initially 
present were not determined, but the final numbers are sufficiently 
low to indicate a satisfactory treatment. Furthermore, it was 
shown that increasing the concentration of the chlorine beyond a 
certain limit has very little effect on the residual bacteria. The 
results of this work are summarized in Table 7. 


Indian government resolution on the working of septic tanks. Calcutta, January 6,1906. 




METHODS OF DISINFECTION. 


25 


Table 7. — Disinfection of septic sewage by chloride of lime at Bengal, India. 


Available 
chlorine (in 
parts per 
million). 

Number of 
tests. 

Number of 
samples. 

Bacteria re¬ 
maining 
(average 
number per 
cubic centi¬ 
meter). 

20 

9 

31 

33 

30 

7 

24 

10 

40 

8 

28 

48 

60 

7 

25 

52 


Experiments in Ohio .—In 1907 the Ohio State Board of Health 
studied the disinfection of sewage effluents in cooperation with the 
Bureau of Plant Industry of the United States Department of Agri¬ 
culture. The results, reported by Kellerman,. Pratt, and Kimberly/ 
related.in part to the use of chloride of lime. The average results of 
four series of tests made with this disinfectant are summarized in 
Table 8. 


Table 8. — Disinfection of effluents with chloride of lime at Lancaster and at Marion, 

Ohio. 

[Kellerman, Pratt, and Kimberly.] 


Series. 

Available 
chlorine 
(in parts 
per mil¬ 
lion). 

Number of bacteria' 
per cubic centimeter 
at 20° C. 

Number of bacteria 
per cubic centimeter 
at 37° C. 

Number of acid form¬ 
ers per cubic cen¬ 
timeter at 37° C. 

Initial. 

Final. 

Initial. 

Final. 

Initial. 

Final. 

A. 

4.0 

130,000 

140 

14,000 

49 

840 

0 

B. 

2.8 

60,000 

1,600 

12,000 

120 

3,000 

0 

C. 

4.1 

225,000 

1,600 

120,000 

390 

16,000 

1 

D... 

6.0 

2,000,000 

700,000 

900,000 

230,000 

70,000 

24,000 


Each series represents a three-day test, during which 18 samples 
were examined in duplicate. Series A was made on the effluent of a 
sand filter at the Boys’ Industrial School, Lancaster, and Series B, 
C, and D on the sand-filter effluent, contact-filter effluent, and septic- 
tank effluent, respectively, at Marion, Ohio. A subsequent study of 
the possibilities of treating the septic tank effluent at Marion led the 
authors to conclude that satisfactory disinfection could be accom¬ 
plished by the use of sufficient bleaching powder to give 25 parts 
per million of available chlorine. 

ELECTROLYTIC CHLORINE PROCESSES. 

Electrolytic manufacture of chlorine . — When an electric current under 
a tension of not less than 2.5 volts is passed through a solution of 
common salt or of calcium or magnesium chloride, chlorine gas appears 


“ Kellerman, K. F., Pratt, R. W., and Kimberly, A. E., The disinfection of sewage effluents for the pro¬ 
tection of public water supplies: Bull. 115, Bur. Plant Industry, U. S. Dept. Agr., 1907. 
























26 


DISINFECTION OF SEWAGE. 


at one electrode and sodium hydroxide or the corresponding alkali 
at the other. In the electrolytic manufacture of chlorine the prod¬ 
ucts of the dissociation are kept apart and are removed from the cell 
as quickly as possible, for if they are allowed to come together again 
they immediately unite and form a hypochlorite. This method of 
manufacturing hypochlorites has been employed in many of the 
so-called “electrolytic disinfection processes.” 

The Webster process . — One of the earliest electrolytic treatments 
was devised by Webster over twenty years ago, when, in 1889, he 
installed an experimental electrolytic plant at Crossness, England, to 
treat London sewage.® In his process crude sewage flowed between 
iron electrodes placed in long troughs, and an electric current was 
passed from one electrode to the other at a tension of only two volts 
and a current density of 0.9 ampere per square foot of electrode. It 
was estimated that the treatment of 1,000,000 gallons of crude sew¬ 
age required the consumption of 240 pounds of iron and 450 kilowatt 
hours of electricity. Sedimentation followed the electrical treatment, 
and a large amount of material was removed. In fact the process 
was virtually one of chemical precipitation, the iron dissolved from 
the electrode being first converted into hypochlorite, or other salt, 
and then being decomposed by the alkali present and precipitated. 
The solution of iron at the positive pole allowed the electrolytic 
reaction to proceed at a lower voltage than that required to liberate 
free chlorine. The results shown in Table 9 are the-averages of 20 
analyses, and they indicate the degree of purification obtained by 
the Webster process. 


Table 9. — Analyses of London sewage before and after treatment by the Webster process. 



Parts per million. 

Initial. 

Final. 

Suspended solids.. . 

333.5 

43.4 
5.0 

12.4 

15.6 

32.2 

2.0 

5.2 

Nitrogen as free ammonia... . 

Albuminoid ammonia. 

Oxygen consumed. . 



Though the process was originally conducted as a chemical pre¬ 
cipitation, credit is due Webster for first pointing out the disinfecting 
value of the hypochlorites that are formed. T. M. Drown, 6 comment¬ 
ing on the process, observed that the American Public Health Asso¬ 
ciation recognized the value of hypochlorites as early as 1888, and 
that their electrolytic manufacture was nothing new. Nevertheless 
its application to sewage was new and gave promise of success. 


a The Engineer, London, vol. 67,1889, p. 261; also Eng. News, vol. 21,1889, p.338; vol. 22,1889, p. 388. 
b Jour. New England Waterworks Assoc., vol. 8, 1894, p. 135; also Eng. News, vol. 31, 1894, p. 236. 

















METHODS OF DISINFECTION. 


27 


In a later paper before the British Medical Association® Webster 
called attention to the possibilities of electrolyzing sea water and thus 
laid the foundation for the many later processes that are based on that 
principle. A plant was later installed at Bradford, England, and in 
1890 Doctor McLintock stated that 70 per cent of the putrescible 
organic matter of the sewage had been removed. 6 

Fermi, c after having investigated the process at the hygienic 
institute at Munich, concluded that the process is one of chemical 
precipitation and that it is more expensive than the lime process, 
and similar conclusions were reached by Konig and Remele/ 

The Woolf 'process — Electrozone. —Woolf’s process differed from 
Webster’s in that strong brine was electrolyzed, and the resultant 
chlorine and caustic soda were allowed to recombine to form sodium 
hypochlorite. The hypochlorite solution was then added to the 
sewage or water to be treated. In the spring of 1893 a plant of this 
kind was installed under the direction of the health department of 
New York City for treating the sewage of about 31 dwellings at 
Brewster, N. Y., e a village situated on a small stream, the waters 
of which discharge into Croton Lake. The experiment was con¬ 
sidered so successful that the health department installed a similar 
plant at the same place to discharge hypochlorite solution into Ton- 
netta Creek/ Sixteen hundred pounds of salt per million gallons 
of sewage were used, and the plant required an electric current of 700 
amperes at 5 volts tension. This seems to have been the first plant 
established for the specific purpose of destroying bacteria; before 
that time the removal of organic matter had been the aim. An 
electrozone plant installed at Maidenhead, England, in 1897, was 
examined by Rideal, Robinson, and Kanthack in 1898. The bac¬ 
tericidal action was marked and the effluent was found to contain 
4 but few bacteria; this plant was not, however, continued 0 in opera¬ 
tion. At Havana, Cuba, the Woolf process was employed for pre¬ 
paring a disinfectant solution to be used for treating the streets and 
the harbor. 

The Hermite process. —The Hermite system differs from the Woolf 
system only in minor details. In reports on the process great stress 
is laid on the presence of magnesium hypochlorite in the electrolyzed 
solution, and there is much evidence that magnesium hypochlorite, 
owing probably to its lesser stability, is a more active agent than 
other hypochlorites. In later years this same fact has been observed, 

a Eng. News, vol. 22, 1889, p. 388. 
b Brit. Med. Jour., vol. 2, 1890, p. 498. 
c Arch. f. Hyg., vol. 13,1891, p. 207. 
d Arch. f. Hyg., vol. 28, 1897, p. 185. 

« Eng. News, vol. 30, 1893, p. 41. 

/ Eng. Record, vol. 29, 1894, p. 110; Elec. Eng., vol. 18, 1894, p. 101. 
g Rideal, S., Sewage and its purification, 3d ed., New York, 1906, p. 185. 



28 


DISINFECTION OF SEWAGE. 


and it has been the basis of fanciful claims for patented processes, 
whose owners have invoked hypothetical oxides of chlorine to explain 
the results. A commission appointed by the London Lancet to 
investigate the Hermite process found that the solution obtained was 
similar to ordinary hypochlorite ® in its chemical properties. A plant 
installed at Worthing, England, was investigated in 1894 by Kelly, 6 
who reported that the hypochlorite solution contained from 0.22 to 
0.75 gram per liter of available chlorine. The claim made for this 
solution — that mixed with equal parts of sewage it would instantly 
kill all germs—was not substantiated. A plant employing the 
Hermite process, established at. Havre, France, in 1893, was investi¬ 
gated by a commission appointed by the imperial board of health 
of Germany and by one sent from Paris by the council of hygiene, 
and both bodies reported adversely. Other Hermite plants were 
installed at various places, but were finally abandoned. At Poplar, 
England, the Hermite solution is prepared on a large scale, and it is 
used for general disinfecting purposes as well as for street watering. 

The oxychloride process .—The oxychloride process differs from the 
Hermite and Woolf processes only in matters of detail in the elec¬ 
trolytic cell. Greater efficiency than the older processes in the pro¬ 
duction of hypochlorites is claimed for it. At Guilford, England, 
Rideal made a test of the effect of oxychloride treatment on raw and 
septic sewages and on effluents of primary, secondary, and tertiary 
contact beds. The summary of his results given in Table 10 shows 
what was accomplished. The results, especially in respect to the 
removal of B. coli, are all that can be desired. 

Table 10 .—Summary of Rideal’s experiments on the use of oxychloride at Guilford, 

England. 


Source of sample. 


Septic effluent... 

Effluent from first 
contact bed. 

Effluent from sec- 
on d contact 
bed 

Effluent from 
third contact 
bed. 


Avail¬ 

able 

chlo¬ 

rine 

parts 

per 

mil¬ 

lion. 


30 

50 

70 

25-44 

20 

20 

10.6 


2.5 

2.5 

.5 


. Time of 
contact. 


4.3 hours.. 

-do- 

. do... 

1 to 4 hours 
40 minutes 

2 hours. 


1 hour 
4.5 hours. 
0.5 hour.. 
4.5 hours. 


Number of organisms per cubic centimeter. 


23,000,000 
23,000,000 
23,000,000 
2,500,000-4,500,000 


.,000,000-2,000,000 


50,000 
20 
10 
20-600 


B. enteritidis. 


1,000,000 
1,000,000 
1,000,000 
100,000-1,000,000 
100,000 


1,000,000 


1,000-10,000 


1,000-10,000 


Final 

less Initial, 
than— 


1.0 

.2 

.2 

1.0-.2 

.2 


.2 


1,000 
1,000 
1,000 
10 - 1,000 


20-100 
10 - 1,000 


10-100 
"l(M 00 


Final 

less 

than— 


10.0 

.2 

.2 

1.0-.2 


.2 

.2 


1.0 

0.2 


a Lancet, vol. 1, 1894, p. 1321. 


& Public Health, vol. 6,1894, p. 261. 




































METHODS OF DISINFECTION. 


29 


Examinations of the few organisms remaining in the sewages and 
effluents after treatment showed them to be largely organisms of 
the hay bacillus group—aerobic spore-forming bacteria which are 
probably beneficial in the further oxidation of the organic matter. 
Absolute sterilization required very high concentration of chlorine. 

COPPER AND ITS COMPOUNDS. 

Moore and Kellerman, in 1904, suggested that copper sulphate be 
used in water sterilization.® Since that time a great deal of experi¬ 
mental work has been done, mainly in connection with water. The 
more important experiments, with sewage will be outlined. 

Johnson and Copeland * 6 at Columbus, Ohio, in 1904, in their work 
with trickling filter effluents, obtained the results given in Table 11. 

Table 11.— Disinfection of trickling filter effluents with copper sulphate at Columbus, 

Ohio. 


[Johnson and Copeland.] 



Copper 
sulphate in 
parts per 
million. 

Reduction in bacteria 
(per cent). 


In 3 hours. 

In 24 hours. 

First series, average of 3 sets. 

1 '» 

20 

10 

| 20 

{ 40 

90.0 

98.0 

98.5 

40.0 

60.0 

88.0 


99.9 

QQ 

Second series, average of 3 sets. * 


yy. uo 

99.96 

99.7 

99.9 

99.95 




They found the action of copper sulphate to be most rapid during 
the first hour. They estimate the cost for chemicals alone at $5 per 
million gallons of effluent treated with 10 parts per million of copper 
sulphate, and $10 if treated with 20 parts per milli on—an expense 
which they consider prohibitive. 

The use of copper sulphate as a disinfectant was more thoroughly 
investigated by Johnson at Columbus in 1905. c Experiments were 
made with crude sewage, and with effluents from trickling, contact, 
and sand filters. The effect of temperature, of organic matter, and 
of alkalinity on the efficiency of the process were determined and 
special studies were also made to determine the effect of the treatment 
on colon and typhoid organisms. The principal results with the 
various sewages treated are summarized in Table 12. 

“U. S. Dept. Agr., Bureau of Plant Industry, Bull. 64, 1904. 

6 Jour. Infect. Diseases, Suppl. No. 1, 1905, p. 327; also Reports and papers. Am. Pub. Health Assoc, 
vol. 30, pt. 2, p. 327. 

e Report on sewage purification at Columbus, Ohio, 1905, p. 471; also Jour. New England Waterworks 
Assoc., 1905, p. 474. 














30 


DISINFECTION OF SEWAGE. 


Table 12. — Results of disinfection of sewage and effluents with copper sulphate at Colum¬ 
bus, Ohio; total number of bacteria remaining after contact periods of one hour and of 
three hours. a 

[Johnson.] 


Copper 
sulphate 
(in parts 
per 

million.) 

Series A. 

1 hour. 

Series B. 

Series C. 

Series D. 

1 hour. 

3 hours. 

1 hour. 

3 hours. 

1 hour. 

3 hours. 

1,000 

3,000 

1,000 

240 

430 

60 

2,100 

280 

200 


5,000 

600 

1,100 

130 

6,000 

1,400 

100 

9,000 

6,000 

500 

2,100 

230 

5,500 

1,800 

40 


3,400 

700 

1,200 

230 

6,500 

1,300 

20 

14,000 

11,000 

1,900 

3,500 

600 

13,000 

2,600 

10 


21,000 

4,500 

7,500 

1,200 

20,000 i 

5,000 


Bacteria per cc. 


a Series A. Crude sewage; initial number.......... 1,200,000 

B. Effluent of sprinkling filter... 1,000,000 

C. Effluent of contact filter .... .... . 400,000 

D. Effluent of sand filter..... 280,000 


Each series is the average of from three to five sets of tests. 

Longer periods of contact up. to twenty-four hours gc.ve results of 
no additional significance, except that when the lower concentrations 
of copper were used the number of bacteria showed a decided increase 
after about the third hour, an indication that the copper was removed 
from solution, probably by combination either with the organic mat¬ 
ter or with carbonic acid. Subsequent increase in number of bac¬ 
teria is a phenomenon of frequent occurrence in disinfection work. 
It is in fact neither possible nor desirable to prevent perpetually the 
feeding of bacteria on the organic matter of the disinfected effluents. 
The significant fact in Johnson’s tests is that the bacteria originally 
present were practically eliminated, and it may safely be assumed 
that under conditions existing in a stream there would be no multi¬ 
plication of the typhoid or other pathogenic bacteria. 

Two important facts were brought out in this study. The disin¬ 
fection obtained with 100 parts per million of copper sulphate is, for 
practical purposes, but little better than that obtained with 10 parts, 
and results obtained in a one-hour contact are practically as good as 
those obtained in a three-hour contact. Johnson’s work was done at 
summer temperature, and in order to determine the effect of tem¬ 
perature on the germicidal action parallel experiments were con¬ 
ducted at 5° and at 20° C. It was found in general that a result 
which could be obtained at the higher temperature in thirty minutes 
would be attained at the lower temperature in about three hours. 
This is a point that has usually been overlooked, and it is of special 
significance in practical operation, as effluents in the northern lati¬ 
tudes would have a temperature of about 5° C. during much of the 
year. It is interesting to note that temperature has little effect on 
the germicidal efficiency of hypochlorites. 






















METHODS OF DISINFECTION. 


31 


The combined effect of organic matter and alkalinity was deter¬ 
mined by treating the undiluted effluent and the same effluent after 
being diluted 1 to 1 and 1 to 2 with tap water. The effect was 
decidedly noticeable in the weaker concentrations of copper sulphate, 
and much less so where the concentrations of copper were excessive. 
The results with 10 parts of copper sulphate per million are summa¬ 
rized in Table 13. 

Table 13. — Effect of organic matter and of alkalinity on the germicidal properties of 
copper sulphate: per cent removal of bacteria at end of one hour’s contact with 10 parts 
per million of copper sulphate. 

[Johnson.] 


Source of sample. 

Dilution. 

0 

1:1 

1:2 

Crude* scwAgp . 

96.1 

97.5 

98.5 

Trickling filter effluent. 

97. 3 

99.0 

99.1 

Sand filter affluent . 

70 

86 

91 






The efficiency of copper sulphate as a disinfectant was investigated 
in 1906 at the Sanitary Research Laboratory of the Massachusetts 
Institute of Technology by treating the effluent from an 8-foot trick¬ 
ling filter. Table 14 gives the average results divided into two 
periods to show the effect of temperature of the effluent. At the 
conclusion of the tests a composite sample of the sediment drawn 
from the sedimentation tank during the experiment was analyzed. 
The copper contained in the sediment accounted very closely for the 
total amount of copper added, a fact that makes it apparent that 
little copper left the tank in soluble form. 


Table 14. — Disinfection of trickling filter effluent with copper sulphate at Boston, Mass.; 

average results. 



Copper 
sulphate 
(in parts 
per 

million). 

Temper¬ 

ature. 

Number of bacteria per 
cubic centimeter. 

Number of B. coli a per 
cubic centimeter. 

Period. 

Initial. 

Final. 

Per 

centre- 

moved. 

Initial. 

Final. 

Per 

centre- 

moved. 

1906. 

October 13—31 

4 

°F. 

56 

230,000 

14,000 

94.0 

44,000 

640 

98.5 

November 3-19. 

4 

46 

250,000 

51,000 

80.0 

48,000 

770 

98.4 

November 21-December 10... 

8 

43 

240,000 

5,000 

97.9 

32,000 

390 

98.8 


a Jackson’s bile media used. 


The work of Kellerman, Pratt, and Kimberly in Ohio during the 
winter of 1906-7 consisted mainly of experiments with copper sul¬ 
phate as a disinfectant. Their investigations, probably the most 
comprehensive ever made with copper sulphate, were conducted under 
actual working conditions and with several kinds of effluent. The 
76474— irr 229—09-3 






























32 DISINFECTION OF SEWAGE. 

chemical composition of the water, particularly in regard to its hard¬ 
ening constituents, was found to exert an important influence on the 
results. Their original paper® on the. subject contains complete 
chemical analyses of the effluents treated. Table 15, containing a 
brief summary of average results calculated from the original tables, 
shows in a general way what was accomplished. The authors con¬ 
cluded that copper sulphate is not so efficient as chlorine compounds, 
is more seriously affected by carbonates, and is much more expensive. 


Table 15 .—Disinfection of effluents with copper sulphate in Ohio. 
[Kellennan, Pratt, and Kimberly.] 


Series. 

Copper 
sulphate 
(in parts 
per mil¬ 
lion.) 

Number of bacteria 
per cubic centi¬ 
meter at 20° C. 

Number of bacteria 
per cubic centi¬ 
meter at 37° C. 

Number of acid 
foemers per cubic 
centimeter. 

Initial. 

Final. 

Initial. 

Final. 

Initial. 

Final. 


5 

6,000,000 

1,200,000 

140,000 

120,000 

6,000 

600 


6.7 

250/000 

70,000 

19,000 

10,000 

6,000 

900 


10 

60,000 

16, COO 

47,000 

10,000 

2,600 

1,000 

A 

13 

65,000 

23,COO 

37,000 

5,500 

4,600 

600 


20 

120,000 

20,000 

8,000 

11,000 

1,600 

no 


29 

60,000 

3,900 

24,000 

6,000 

1,200 

10 


40 

160,000 

8,500 

75,000 

4,700 

5,500 

no 


67 

200,000 

34,000 

81,000 

14,000 

7,000 

700 


7.3 

110,000 

23,000 

12,000 

1,800 

600 

48 

B... 

14 

' 75,000 

9,500 

16,000 

600 

75 

0 


22 

55,000 

7,000 

5,500 

200 

75 

0 


6.5 

390,000 

110,000 

170,000 

41,000 

8,000 

3,000 

Q 

15 

230,000 

65,000 

42,000 

16,000 

5,000 

2,200 


40 

29,000 

6,000 

30,000 

8,500 

5,500 

750 


116 

84,000 

7,000 

28,000 

2,000 

5,000 

560 


A. Effluent of contact filter at Westerville, Obio. 

B. Effluent of sand filter at Lancaster, Ohio. 

C. Effluent of sand filter at Marion, Ohio. 


MISCELLANEOUS METHODS. 

PERMANGANATES. 

Potassium permanganate and sodium permanganate have been 
used for the oxidation of organic matter in streams. At London 
when the Thames becomes extremely foul during low-water periods, 
sodium permanganate is added to its waters in order to destroy odors 
and putrescible material, but the treatment undoubtedly results in 
partly sterilizing the water. It is claimed that the germicidal action 
is not sufficiently great to interfere with the normal oxidizing changes 
in the stream. The use of permanganates has been proposed for ren¬ 
dering effluents of chemical precipitation plants nonputrescible. 

“amines” process. 

The so-called “amines” process was developed by H. Wollheim in 
England. It is claimed that trimethylamine treated with lime or 
other alkali produces a very poisonous substance. Herring brine is 


a Kellennan, K. T., Pratt, R. W., and Kimberly, A. E.: The disinfection of sewage effluents for tbfl 
protection of public water supplies, Bull. 115, Bur. Plant Industry, U. S. Dept. Agr. 1907. 



























METHODS OF DISINFECTION. 


33 


used to supply the amine. A large excess of lime is added and the 
mixture is used to precipitate crude sewage. Klein made a test of 
the process at West Horn in 1889, and found that a clear, nonpu¬ 
trescent, sterile effluent could be obtained. Similar results were 
obtained at Wimbledon, 0 where 768,000 bacteria per cubic centime¬ 
ter in the sewage were completely removed. The sludge is also 
nonputrescible. The process does not seem to have been further 
developed. 

SODIUM BENZOATE AND OTHER ORGANIC COMPOUNDS. 

Sodium benzoate is supposed to possess powerful germicidal prop¬ 
erties, and its use as a disinfectant for sewage was suggested to the 
writer. It would apparently have the distinct advantage over such 
other disinfectants as chlorine and copper salts of not combining with 
organic matter, thus rendering all the disinfectant added available for 
a long time. Sodium benzoate was applied regularly to a trickling 
filter effluent at Boston during March and April, 1907. An addition 
of commercial benzoate at the rate of 0.8 part per million for twenty- 
six days gave an average reduction of total bacteria from 140,000 to 
54,000 per cubic centimeter, or a 62 per cent removal. Doubling the 
amount of benzoate increased the efficiency somewhat, the average 
reduction then being from 370,000 to 84,000 per cubic centimeter, or 
78 per cent. The cost of benzoate for the treatment is $1 per million 
gallons for 0.8 part and $2 for twice that quantity. Obviously this 
substance is not an efficient disinfectant in sewage work. 

Other organic substances, such as the phenols and the coal-tar 
products, were suggested, but in general their high cost eliminates 
them from consideration. The possibility of after effects must also 
be considered. There is a certain advantage in the use of a sub¬ 
stance which is itself used up in the reaction or which is converted 
into harmless compounds. On the other hand, the organic disin¬ 
fectants are extremely powerful and there is always a chance of the 
discovery of some new compound with the requisite germicidal prop¬ 
erties and the low cost that will make it the ideal sewage disinfectant. 

SUMMARY OF DISINFECTION METHODS. 

Two of the methods of disinfection that have been mentioned 
appear not to have been sufficiently investigated, namely, disinfec¬ 
tion by heat and by organic compounds. The heat method holds a 
reasonable possibility that sufficient ammonia may be recovered to 
pay for the necessary heating, but actual trial is essential for con¬ 
vincing proof. A systematic study of organic compounds as disin¬ 
fectants has yet to be made. 


a Interim Rept. Royal Sewage Commission, 1901, p. 304. 





34 


DISINFECTION OF SEWAGE. 


Of the disinfectants that have been sufficiently investigated, 
chlorine compounds and copper salts alone appear to be applicable 
to the sewage problem. Moreover, a detailed study of results on a 
cost basis leaves no doubt that the efficiency of chlorine is much 
greater than that of copper. Even if the prices of these two mate¬ 
rials were more nearly equal, many facts favor the use of chlorine. 
Both reagents unite with organic matter, but chlorine unites by oxi¬ 
dizing the organic matter, thus rendering it less putrescible, while cop¬ 
per precipitates it. The compounds formed by the. copper tend to 
protect the solid particles from further action, and the diffusion of cop¬ 
per ions through such a precipitated copper envelope must necessarily 
be slow. As no such action occurs with chlorine compounds, the 
penetration of the chlorine into solid particles must be much more 
complete. Chlorine in the form of bleaching powder is to some 
extent a by-product, is very cheap, and will probably become cheaper 
as the methods of production are improved; on the other hand, 
copper is a staple, the price of which is likely to increase. Sewages 
may be found of such chemical composition that treatment with 
copper will be more effective than that with chlorine, but in general 
chlorine compounds are to-day by far the most economical and the 
most efficient disinfectants available in sewage work. 

EXPERIMENTAL INVESTIGATIONS. 

HISTORY. 

In connection with general investigations in sewage disposal, 
which have been conducted since 1903 at the sanitary research 
laboratory and sewage experiment station of the Massachusetts 
Institute of Technology, the subject of the chemical disinfection of 
sewage and sewage effluents has been extensively studied during the 
past two years. In the spring of 1906 a cooperative arrangement 
between the United States Geological Survey and the institute made 
it possible materially to enlarge the scope of the work. 

With a view also of gaining increased knowledge of the practical 
workings of the processes that had been developed at Boston, an 
arrangement was made with the state sewerage commission of New 
Jersey, through Boyd McLean, secretary, under which experiments 
were begun at Red Bank, N. J. The state sewerage commission 
later continued that work for its own information, retaining the 
writer in charge, and through the courtesy of the commission the 
results of the entire work are here presented. During the sum¬ 
mer of 1907 the experiments were placed under the immediate 
charge of F. E. Daniels, to whose faithful efforts their successful 
completion is in large part due. Mr. Daniels was assisted at Red 
Bank by H. S. Crawford, and also did much of the work at Boston 
during 1908. 


EXPERIMENTAL INVESTIGATIONS. 


35 


In December, 1907, the scope of the investigation was still further 
broadened by a cooperative agreement between the United States 
Geological Survey and the sewerage commission of Baltimore, of which 
Brig. Gen. Peter Leary, U. S. Army, retired, is chairman, and Calvin 
W. Hendrick is chief engineer. Experiments on a somewhat enlarged 
scale were immediately undertaken at the Walbrook testing station 
of the Baltimore sewerage commission, and they were continued up 
to July, 1908, at which date the agreement with the Geological Survey 
lapsed. The work at Baltimore was under the immediate charge 
of Ezra B. Whitman, division engineer of the disposal division, to 
whom the writer is under the greatest obligation for its successful 
outcome. Intimately associated with that work also and responsi¬ 
ble in large measure for the results have been Charles A. Emerson, jr., 
assistant division engineer, Henry G. McRae, chemist, and Edward 
G. Birge, bacteriologist. Dr. R. P. Cowles, of Johns Hopkins Uni¬ 
versity, and William T. Carpenter, Leyland Whipple, and Marvin H. 
Lillis, have also been associated with this work from time to time in 
connection with special lines of investigation. 

Through the courtesy of A. G. Paine, jr., president, and J. R. 
Crocker, superintendent, of the McDonald Electrolytic Cell Com¬ 
pany, of New York, an electrolytic chlorine cell having a capacity 
of 22 pounds a day of chlorine gas was placed at the disposal of the 
writer, and it proved to be of great assistance in the work, as a large 
supply of gaseous chlorine was thus made available for special experi¬ 
ments. The machine was used directly in disinfection and also in the 
preparation of a series of chlorine compounds that were studied. 

BACTERIOLOGICAL METHODS AND EXPRESSION OF RESULTS. 

The bacteriological methods employed in the different parts of this 
work have been kept as nearly uniform as possible in order that the 
results may be strictly comparable. In general, the methods have 
been those recommended by the committee on standard methods of 
water analysis of the American Public Health Association, labora¬ 
tory section. Since the publication of those methods,® a new pre¬ 
sumptive test for the colon bacillus has been described by Jackson 6 
and it has been used throughout this work. The medium employed 
in this test consists of ox bile to which lactose has been added. Its 
accuracy in sewage work has never been carefully studied, but 
recent studies by Prescott and Winslow c have confirmed the earlier 
statement of Jackson that in water work this presumptive test 
gives results which are similar to those obtained in complete B. coli 

a Jour. Infect. Dis. Suppl. 1, Chicago, 1905. Repts. and Papers American Pub. Health Assoc., vol. 30, 
pt. 2,1905. 

b Biological studies by the pupils of William Thompson Sedgwick, p. 292, Boston, 1906. 

c Elements of water bacteriology, 2d ed., New York, 1908, p. 149. 




36 


DISINFECTION OF SEWAGE. 


determinations. It may fairly be assumed, therefore, that the B. 
coli results reported in the present paper represent a group of bac¬ 
teria, including practically all the B. coli actually present, together 
with certain other bacteria which probably are not more than 10 
per cent of the whole number reported. The study of such a group 
gives valuable information concerning the probable elimination of 
typhoid and other pathogenic bacteria, while the additional infor¬ 
mation to be obtained from a complete determination of the B. coli 
actually present is not deemed sufficient to warrant the additional 
labor involved. At Boston and at Baltimore counts of the bacteria 
on gelatin at 20° C. and on litmus lactose agar at 37° C. and 
counts of the acid-forming bacteria at 37° C. were recorded. The 
latter group includes the B. coli, and some bacteriologists believe 
that the number of acid-forming bacteria is a more accurate 
indication of the numbers of colon bacilli than the presumptive 
tests. It is indisputable, however, that the group of acid¬ 
forming bacteria is large and that in sewage work at least it 
bears only a general relationship to B. coli. An additional group, 
the bacteria that liquefy gelatin, is also included in the Boston results, 
though the additional information afforded by including this group 
is slight. Inasmuch as adequate facilities for complete bacteriological 
work were not available at Red Bank, the total count at 20° C. and 
the presumptive B. coli tests alone were made. Examinations of the 
trickling filter effluent before and after treatment have been made at 
Boston on five or six days in each week. At Red Bank and at Bal¬ 
timore two samples were tested each working day for B. coli and 
three samples were plated for the bacterial counts. 

Considerable thought has been given to the manner of expressing 
results. It would obviously be inadvisable to attempt to tabulate 
the results of all examinations made, because such tables would be 
too unwieldy for general use. On the other hand, the use of aver¬ 
ages, especially if they cover long periods, is open to the serious 
objection that a few bad results are easily covered up in general aver¬ 
ages, and it is particularly necessary, in a process of this kind, to 
show that the results are not only satisfactory in general, but that 
they are fairly uniform. To cite a familiar illustration, a poor 
marksman’s shots might be placed symmetrically about the center 
of the target and in that sense their-average position might be as 
near the bull’s-eye as the average position of better-placed shots. 
The actual deviations from the average position would indicate the 
character of the shooting. Similarly, in disinfection two series of tests 
might give the same average, even if one series were composed of 
fairly uniform results and the other series of very erratic ones. In 
other words, the deviations from the average are fully as significant 
as the average figure itself. Accordingly, the routine work is reported 


INVESTIGATIONS AT BOSTON. 


37 


in the form of weekly averages. The total bacterial counts are 
accompanied by a statement showing the variations of the individual 
results and the variations of the individual bacterial removals from 
the average bacterial removal. In this way the record of what was 
actually accomplished is stated in the most compact form. 

A new method of recording B. coli results is also employed. The 
procedures used in the determination of this organism do not permit 
a quantitative count of the number really present, but show merely 
the presence or the absence of the bacillus. Quantitative results are 
obtained in a rough way by applying the test to various dilutions of 
the sample. For instance, in a disinfected effluent the test is applied 
on 0.1, 0.01, and 0.001 cubic centimeter, respectively, and the highest 
dilution giving a positive test is recorded. Hitherto a bare state¬ 
ment of the results is all that has been attempted in the way of giving 
quantitative significance to this test. But it has been shown ° that 
where as many as fifty or one hundred results of that character from 
the same source are available, the probability law of distribution may 
be applied in order to estimate the average number of organisms 
present. The most probable value for the number of B. coli present 
in each sample is indicated by the reciprocal of the highest dilution 
giving a positive test, and while that figure may be far from the cor¬ 
rect one for any one sample, the average of fifty estimates of this 
character closely approximates the actual value. The case is analo¬ 
gous to the well-known facts in relation to the expectancy of life at 
any age; an expectancy of life as given in the actuarial tables has 
but little weight when applied to any one person, but in the average 
of a large number of persons it approximates the truth. The num¬ 
ber of B. coli present in a sample has therefore been recorded in this 
way. Individual results have but little weight and weekly averages 
must be properly interpreted, but monthly averages are probably 
very near the average numbers. 

INVESTIGATIONS AT BOSTON. 

SCOPE AND CHARACTER OF EXPERIMENTS. 

Since the beginning of the work at Boston in February, 1906, 
various phases of the disinfection problem have been under investi¬ 
gation. The following summary of the studies shows their scope and 
character: (a) Preliminary experiments with chloride of lime; (b) studies 
of the comparative efficiencies of chloride of lime, sulphate of cop¬ 
per, and sodium benzoate; (c) small-scale experiments to determine 
the comparative efficiencies of free chlorine, commercial hypochlo¬ 
rites, electrolytic hypochlorites, chlorates and perchlorates, and the 
hypochlorites of several bases; (d) studies of the comparative effect 

“Phelps, E. B., A method tor calculating the numbers of B. coli from the results of dilution tests: Am. 
Jour. Pub. Hyg., vol. 18, 1908, p. 141. 




38 


DISINFECTION OF SEWAGE* 


of hypochlorites on the typhoid and the colon bacilli; ( e ) routine 
disinfection of 5,000 gallons a day of trickling-filter effluent with 
chloride of lime; (/) disinfection of crude sewage with chloride of 
lime in 700-gallon tests. The studies on copper sulphate and sodium 
benzoate have already been discussed (pp. 29-32). The remaining 
divisions of the work are reviewed in the succeeding pages. 

PRELIMINARY EXPERIMENTS WITH CHLORIDE OF LIME. 

The results of some preliminary experiments in the spring of 1906 
have already been published, 0 but as they have important bearing 
on the present work, a brief review of them is here presented. The 
experiments were made with chloride of lime and the effluent of a 
trickling filter in which Boston sewage was being treated. They 
consisted of 23 bottle tests in which the available chlorine that was 
added varied from 0.25 to 100 parts per million, and the time of 
contact from thirty minutes to twenty-four hours. They were under¬ 
taken with the object of establishing practical working limits for 
available chlorine and contact period for future experiments, and 
particularly to determine whether the English and Continental prac¬ 
tice of adding large, almost prohibitive, amounts of chlorine is justi¬ 
fied by the results. 

This preliminary study indicated that an effluent similar to the 
one used can be deprived of 95 per cent to 98 per cent of its total 
bacteria in two hours by the application of chloride of lime in con¬ 
centrations having 2 to 5 parts per million of available chlorine. 
The impracticability of attempting to get much better results was 
well shown, for complete sterilization was never accomplished, though 
concentrations of chlorine up to 100 parts per million and storage 
periods of twenty-four hours were employed. It was found unnec¬ 
essary, in brief, to use more than 5 parts of chlorine or to treat for 
over two hours. 

It is a well-known fact that, in all processes involving the destruc¬ 
tion of bacteria, it is comparatively easy to kill the first 95 per cent 
of the germs and very difficult to destroy the remaining 5 per cent. 
This phenomenon of the “ resistant minority/ 7 as Whipple terms it, 
is common to all kinds of sterilization, whether it be by heat, cold, 
light, chemicals, or other means. It is therefore more practical to 
determine how far disinfection may be carried at a reasonable expend¬ 
iture than to attempt the ideal complete sterilization. To state a 
concrete example, it might happen that the pathogenicity of an 
effluent could be reduced 96 per cent by the expenditure of a certain 
sum of money, 97 per cent by the expenditure of twice that sum, 
and 99 per cent by the expenditure of five times that sum. The 
first reduction might represent a feasible plan, and the last a pro- 

a Phelps, E. B., and Carpenter, Wm. T., The sterilization of sewage filter effluents: Tech. Quart., vol. 
19, 1906, pp. 382-403. 




INVESTIGATIONS AT BOSTON. 39 

hibitively expensive one. It is plainly more desirable at the outset 
to determine the relations between the various factors, such as con¬ 
centration of disinfectant, time of contact, cost, and efficiency, and 
to establish certain working limits with reference to final costs, 
than to attempt the ultimate destruction of all germs regardless of 
the practical limits that necessarily exist. The preliminary experi¬ 
ments were undertaken with the idea of fixing these practical work¬ 
ing limits. For the first time in the history of the subject the disin¬ 
fection of sewage filter effluents arid even of crude sewage itself was 
shown to be practicable and feasible, yielding results commensurate 
with the cost when compared with the results and costs of other 
purification processes. 

DISINFECTION OF TRICKLING-FILTER EFFLUENT. 

The most important work at Boston consisted in the routine 
disinfection of the combined effluents of two trickling filters. Each 
filter is 8 feet deep and has a surface area of 50 square feet. One is 
composed of crushed stone from 1 inch to 1^ inches in mean diam¬ 
eter; the other of stone from to 2 inches in diameter. The com¬ 
bined effluents, amounting to 5,000 gallons in twenty-four hours, 
were brought from the underdrains of the outdoor trickling filters 
to the filter house through a short length of iron pipe. Within the 
filter house the liquid was conducted by an open trough to the central 
channel of a sedimentation tank built on the Dortmund plan. It 
then passed downward nearly to the bottom of this conical tank, 
entered the main body of the tank, and, rising with constantly 
decreasing velocity, finally overflowed at the surface. The tank 
was designed to give a two-hour storage period. The disinfectant 
solution was made Up in a 50-gallon barrel to a strength about one 
hundred times that required in the final mixture. A small orifice- 
box working under constant head was connected with the barrel 
and was designed to deliver exactly 2 gallons an hour. As the flow 
through the sedimentation tank was a little over 200 gallons per 
hour, this arrangement gave a final mixture of the desired propor¬ 
tions. These rates and volumes were kept constant throughout, 
any change in the amount of disinfectant added being brought about 
by changing the strength of the solution. Readings of a float gauge 
set in the barrel and daily measurements of the flow of the trickling- 
filter effluent into the tank served as checks on the accuracy of the 
dilutions, and together with daily analyses of the strong disin¬ 
fecting solution, gave data from which the actual amount of dis¬ 
infectant added during any required period of time could be com¬ 
puted. Observations of the rate of flow of the disinfectant solution 
into the tank were made at hourly intervals for a period of three 
hours preceding the taking of samples, and the actual concentration 
of the disinfectant corresponding to the sample in question was 


40 


DISINFECTION OF SEWAGE. 


calculated from these observations. The disinfectant solution flowed 
into the open trough previously mentioned, where it mingled with 
the effluent. Further opportunity for mixing occurred during the 
passage down the central channel of the sedimentation tank. Sam¬ 
ples for bacterial examination were collected with the usual pre¬ 
cautions, before the effluent had reached the wooden trough into 
which the disinfectant solution was run and also at the final outlet 
of the sedimentation tank after the combined effluent and disinfec¬ 
tant had been in contact for a period somewhat less than two hours. 
These samples are described as initial and final, respectively. 

The chemical composition of an effluent has an important influence 
on the germicidal value of most disinfectants, particularly chloride 
of lime. The chlorine applied is eventually completely consumed by 
chemical reaction with the organic matter of the effluent, and the 
germicidal action takes place in the interval of time between the 
addition of the chlorine and its final exhaustion by chemical reaction. 
The amount of organic matter present, therefore, practically deter¬ 
mines the amount of chlorine that it is necessary to use. For this 
reason the chemical analyses of the combined effluent are recorded 
by monthly averages in Table 16. The monthly averages of the 
analyses of the disinfected and settled effluent are also included to 
show the chemical character of the effluent after treatment. 

Table 16. — Chemical analyses of trickling-filter effluent at Boston before and after disin¬ 
fection with chloride of lime and sedimentation; monthly averages. 

[Parts per million.] 

INITIAL. 


Month. 

Tur¬ 

bid¬ 

ity. 

Suspended 

solids. 


Nitrogen as 



Oxygen 

consumed.** 

Dis¬ 

solved 

oxy¬ 

gen. 

Total. 

Loss 

on 

igni¬ 

tion. 

Organic. 

Free 

am¬ 

monia. 

Ni¬ 

trites. 

Ni¬ 

trates. 

Total. 

Dis¬ 

solved. 

Total. 

Dis¬ 

solved. 

1907. 












November. 

135 

118 

78 

3.5 

1.5 

15.0 

0.2 

2.0 

40 

32 

8.2 

December. 

155 



5.5 

3.0 

15.5 

0.1 

1.0 

38 

33 

7.8 

1908. 




January. 

140 

92 

69 

5.0 

3.0 

15.0 

0.3 

3.5 

45 

38 

10.0 

February. 

145 

177 

156 

6.0 

2.5 

14.5 

0.1 

3.0 

44 

32 

12.5 

March. 

130 

180 

122 

7.5 

3.5 

15.5 

0.2 

4.0 

48 

35 

7.8 

April. 

195 

313 

192 

■10.5 

5.5 

14.5 

0.4 

5.5 

64 

46 

9.4 

May. 

275 

436 

247 

14.0 

4.5 

14.5 

0.9 

6.0 

70 

35 

7.4 

June. 

155 

174 

96 

11.5 

6.0 

7.5 

0.8 

6.0 

49 

35 

7.1 

Average. 

165 

213 

137 

8.0 

4.0 

14.0 

| 0.4 

4.0 

50 

36 

8.8 


FINAL. 


1907. 












November. 

105 

71 

53 

3.0 

1.5 

14.5 

0.1 

2.0 

28 

27 


December. 

145 



4.0 

1.5 

14.0 

0.1 

2.0 

39 

38 


1908. 









January. 

115 

124 

103 

3.0 

3.0 

13.0 

0.6 

5.0 

49 

41 


February. 

110 

259 

239 

4.5 

2.5 

13.5 

0.3 

4.5 

42 

38 


March. 

125 

125 

82 

9.5 

3.5 

15.0 

0.3 

3.5 

46 

42 


April. 

105 

147 

125 

5.5 

3.5 

14.5 

0.4 

5.0 

44 

40 


May. 

135 

92 

69 

5.0 

3.0 

15.0 

1.0 

5.0 

44 

33 


June. 

95 

51 

43 

7.0 

4.0 

7.5 

0.8 

6.0 

38 

33 


Average. 

115 

124 

102 

5.0 

3.0 

13.5 

0.5 

4.0 

41 

37 



Thirty-minute boiling method. 




































































INVESTIGATIONS AT BOSTON. 


41 


This disinfection experiment was started November 11, 1907, and 
was continued practically without interruption till June 27, 1908, a 
period of thirty-three weeks. The value of such continuous experi- 
iment is obvious. A certain length of time is always required for 
for the establishment of uniform working conditions, not only in the 
personal element but in the tanks themselves. Furthermore, short 
special experiments receive an unusual and perhaps unfair amount 
of care and attention, which is not bestowed on work that has become 
part of the routine, and the natural defects of the processes under 
practical working conditions are, therefore, not always discovered. 
The most important value of long-continued tests, however, lies in 
their being carried out under various seasonal conditions. Fluctu¬ 
ating conditions of temperature and rainfall fundamentally affect the 
sewage and influence the work of the filter. It is consequently of 
prime importance to determine the efficiency of any purification 
process under an extreme range of seasonal conditions. It fortu¬ 
nately happened that the present experiments were continued 
through an unusually cold winter and through the hottest portions 
of an exceptionally hot, dry summer. 

Table 17 contains the results of the experiments, given in weekly 
averages, which are the mean of four to six daily results. During 
the first five weeks the available chlorine added was about six parts 
per million, but during the remainder of the period this concentra¬ 
tion was reduced to between two and four parts without materially 
affecting the results. The average results for the whole period are 
indicated in the last line of the table. 


Table 17 . —Disinfection of trickling-filter effluent with chloride of lime at Boston; weekly averages. 


42 


DISINFECTION OF SEWAGE 


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INVESTIGATIONS AT BOSTON. 


43 


The average efficiencies as measured by the per cent of bacteria 
removed for the whole period and for certain shorter periods, selected 
to illustrate the effect of different concentrations of chlorine and of 
fluctuating conditions of temperature, are shown in Table 18. 

Table 18.— Disinfection of trickling-filter effluent at Boston; summary of bacteriological 
results, averaged by -periods, to show the effect of changes in temperature and in the amount 
of available chlorine. 


Period. 

Temper¬ 

ature. 

Avail¬ 

able 

chlorine, 
in parts 
per mil¬ 
lion. 

Per cent of bacteria removed. 

Bacteria at 20° C. 

Bacteria at 37° C. 

Total. 

Lique- 

fiers. 

Total. 

Acid 

formers. 

■B . coli. 

November 12 to June 27. 

November 12 to December 12... 

January 27 to March 28. 

April 27 to June 27. 

° F 

45 

42 

36 

60 

3.4 

6.3 

3.2 

2.9 

96.8 
99.57 

95.8 
97.1 

98.1 

99.73 

97.7 

98.0 

97.4 

99.81 

96.6 

97.6 

97.3 
99.91 

96.4 
97.9 

99.19 
99.99 
98.5 
99.07 


Comparison of the results obtained in the periods November 12 to 
December 12 and January 27 to March 28 shows that the additional 
bacterial removal gained by increasing the available chlorine from 
3.2 to 6.3 parts per million, although it is' considerable in amount, is 
hardly commensurate with the cost. The effect of temperature on 
the bacterial removal is shown in the periods from January 27 to 
March 28 and April 27 to June 27. The mean temperature is 36° F. 
in one period and 60° F. in the other, while the available chlorine is 
practically the same in both periods, though slightly higher during 
the period of lower temperature. An advantage in favor of the sum¬ 
mer results is noticeable, and it would probably have been a little 
greater with exactly the same amount of chlorine. Yet the results 
show especially that the effect of temperature on efficiency is not 
great. This point is of special significance in comparing the results 
of chlorine disinfection with those obtained with copper. In experi¬ 
ments with copper, temperature has been shown to produce a marked 
effect; in fact, the concentration of copper must be doubled during 
the winter months to maintain the efficiency of the process. The 
bacterial removals in the several groups of bacteria recorded are 
nearly the same, though the removal of B. coli is higher in all experi¬ 
ments. It is probably safe to assume that the removal of the typhoid 
and other pathogenic organisms will be as nearly perfect as the 
removal noted in any of these groups. The per cent removals of 
organisms shown by the counts at 37° C. and by the B. coli results 
probably represent most nearly the per cent removal of typhoid 
bacilli. 

As has been stated, the reliability of a disinfection process depends 
quite as much on the general evenness of the results as on the aver- 



























44 


DISINFECTION OF SEWAGE. 


age efficiency. For that reason the per cent removal of total bacteria 
has been computed for each of the 158 individual tests, and the results 
of the calculations, given in Table 19, show that the per cent removal 
was between 98 and 100 per cent in over half the individual tests, 
and that it was less than 94 per cent in only 15 per cent of the tests. 

Table 19. Relation between individual tests of bacterial removal and the average result 

at Boston. 


Per cent of total 
number of tests.a 

Per cent removal of total 
bacteria. 

Not less 
than— 

Less than— 

54 

98 

100 

19 

96 

98 

12 

94 

96 

6 

92 

94 

2 

90 

92 

3 

85 

90 

2 

75 

85 

2 

65 

75 

Total, 100 

Average, 96.3 


a Number of individual tests, 158. 


The number of bacteria remaining in the disinfected effluent have 
been tabulated in a similar manner, in order that the bacterial quality 
of the final effluent may be properly understood. The results are 
given in Table 20. 

Table 20. Relation between individual bacterial counts and the average result at Boston. 


Per cent of total 
number of tests.« 

Total number of bacteria per 
cubic centimeter. 

Not less than— 

Less than— 

47 

100 

10,000 

23 

10,000 

25,000 

20 

25,000 

50,000 

5 

50,000 

100,000 

3 

100,000 

200,000 

2 

200,000 

250,000 

Total, 100 

Average, 24,000 


a Total number of tests, 158. 


The average of the 158 individual tests of bacterial removal is 96.3 
per cent, as compared with 96.8 per cent removal (see Table 17, p. 42) 
obtained from the average numbers for the whole period. The first 
figure is more nearly correct for estimating the efficiency of the proc¬ 
ess, but it is evident that the last figure, which is more commonly 
employed in such work, gives the average result with sufficient accu¬ 
racy. The somewhat tedious calculation of the individual results 

















INVESTIGATIONS AT BOSTON. 


45 


and their variations has been made only for the total bacteria; but 
the results suffice to show that the average per cent removal of bac¬ 
teria/computed from the average initial and final counts, is practically 
the same as the average of the per cents calculated from the individual 
tests, and that there is a certain variation in the results to the extent 
shown. As to the other groups of bacteria studied, it is sufficient to 
state that similar computations with them gave practically identical 
results, subject to much the same deviations. 

In order to study the effect of varying the time of contact with the 
disinfectant, samples of the trickling-filter effluent were collected from 
the mixing trough after the disinfecting solution had been added. 
After these samples had been well shaken, determinations were made 
of the number of bacteria remaining at the end of ten minutes, fifteen 
minutes, one hour, and two hours, the sample being shaken each time 
before withdrawing the test portion. The number of bacteria in the 
effluent before treatment, as determined on a separate sample, was 
between 250,000 and 1,000,000 per cubic centimeter; but the num¬ 
bers in all tests have been converted to a uniform basis of 1,000,000 
initial bacteria per cubic centimeter for more ready comparison of the 
results in Table 21. 


Table 21. Relation between time of contact and efficiency of disinfection with chloride of 

lime. ® 


Date. 

Number of remaining bacteria per cubic centi¬ 
meter b after contact for — 

♦ 

10 minutes. 

15 minutes. 

60 minutes. 

120 minutes. 

August 6. 


i inn 

100 

1 EO 

9. 

2 500 

A, 1UU 

1QH 

-LOU 

EQ 

10U 

10 . 

10,’ 000 
3,500 
47 000 

A«7U 

270 

K7n 

DO 

7 

40 

11. 

■1 C/t 

14. 

OIVJ 

1,100 

310 

94H 

104 

700 

100 

C7A 

15. 

4* 200 

1 200 


o/u 

1 on 

16. 

IGA 

120 

1 QA 

17 . 

9*800 

800 

luO 

900 

loU 

1 EO 

20. 

400 000 

ouu 
19 OOO 


10U 

5,500 

1 OOO 

21. 

28^000 

1 300 

uuu 

2 100 

7,000 

1 800 

23. 

'230 

A , ouu 
no 

1, UUU 

Q1 



I1U 

oi 

Average. -. ... 

50,000 

1 700 

qeo 

700 

Per cent of remaining bacteria. 

*5.0 

6.17 

you 

0.10 

/UU 

0 07 




a Available chlorine, 5 parts per million. 

b All numbers converted to a uniform basis of 1,000,000 initial bacteria per cubic centimeter. 


The rapidity of the action of the disinfectant is somewhat sur¬ 
prising, and it indicates that long periods of contact are unnecessary. 
It is evident that the greater part of the disinfection is accomplished 
within the first fifteen minutes, and that a contact period of one hour 
is probably ample for practical work. Another feature of interest in 
this experiment is its bearing on the character of the sediment in the 
disinfection tanks. It has been suggested that perhaps such sedi¬ 
ment carries down with it large numbers of the bacteria that are 




























46 


DISINFECTION OF SEWAGE. 


recorded as being removed, though they may be still vigorous within 
the protecting body of solid matter. It is undoubtedly true that 
ordinary sedimentation tanks remove a considerable number of bac¬ 
teria, and the actual removal is doubtless even greater than would 
appear from the data, owing to multiplication during sedimentation. 
But in their slow descent through the liquid, such bacteria are subject 
to the action of the chlorine, the supply of which is constantly renewed 
by the downward movement of the particles. The major portion of 
such sediment is suspended in the liquid for periods varying from 
fifteen minutes to an hour, during which time the chlorine has 
evidently ample opportunity to act. In the series of tests just 
described this sediment was included in the bacterial determinations. 
It is interesting to note, therefore, that the average reduction of 
bacteria at the end of two hours was 99.93 per cent, while the routine 
tests on the same days in the sedimentation tank gave an average 
reduction of 99.96 per cent, results practically identical. The 
sediment is apparently as completely disinfected as the supernatant 
liquid, but naturally an accumulation of sediment at the bottom of 
the tank would soon rid itself of chlorine and permit the multiplica¬ 
tion of the remaining bacteria. Such secondary growth of bacteria, 
however, has no sanitary significance, since it has been satisfactorily 
shown that the pathogens do not develop in such manner outside the 
body. 

These results demonstrate the entire feasibility of satisfactorily 
disinfecting trickling-filter effluents with chloride of lime, and they 
indicate that about 3.5 parts per million of available chlorine and a 
contact period of about one hour are ample for an effluent like that on 
which the experiments were made. A general discussion of the 
results in their practical application to the disinfection problem with 
special reference to costs is given on pages 63-70. 

DISINFECTION OF CRUDE SEWAGE. 

Under certain circumstances it may be necessary and desirable to 
disinfect the crude sewage of a community, though such a procedure 
would not as a rule be considered. It can never take the place of 
purification except by the use of excessive amounts of disinfectant; 
and by destroying the bacteria always present it simply delays the 
natural processes of reduction, allowing them to proceed at a later 
time, possibly after the solid matter has had time to settle to the 
bottom of a stream where the greatest nuisance will be caused by its 
decomposition. It might happen, however, that a sufficiently great 
dilution to preclude anaerobic action and a sufficiently swift current 
or tidal flow to prevent sedimentation would obviate all danger of a 
nuisance except from a bacterial standpoint. Such conditions are, 


INVESTIGATIONS AT BOSTON. 


47 


perhaps, most nearly realized in our seaboard cities, where there may 
be no question of a physical nuisance, but where bacterial pollution 
of the harbor waters may be undesirable for various reasons. 

The disinfection of crude sewage was studied because it was 
believed that experiments with the material might help to solve the 
general problem. The relations between available chlorine, 
amount of organic matter, and germicidal effect suggest that an 
excessive amount of organic matter in raw sewage might affect the 
efficiency of the chloride of lime treatment by combining directly 
with the chlorine and thus preventing its activity. The quality of 
the organic matter might also influence the result; for, as the organic 
matter of crude sewage is more readily oxidizable than that of an 
effluent, it might be expected to consume a greater amount of chlorine 
than the latter in a short time. The experiments with crude sewage 
were undertaken with the object of studying such features and of 
ascertaining the concentrations of chlorine necessary to effect a 
satisfactory removal of the bacteria in crude sewage, having in view 
both the possible disinfection of crude sewage itself and the establish¬ 
ment of a maximum limit of available chlorine in the treatment of 
effluents. 

The details of these experiments differ somewhat from those of the 
effluent tests that have been described. In treating an effluent, 
sedimentation is an advantageous process by itself and can be use¬ 
fully employed as an adjunct in disinfection, but in treating raw 
sewage, the removal by sedimentation of large amounts of organic 
matter in an unoxidized condition is undesirable and is not per¬ 
missible in practice. In the experimental work, therefore, it was 
necessary to prevent sedimentation during the treatment. 

A rectangular tank having a capacity of about 700 gallons was 
employed for the experiments. A 2-inch pipe led from the bottom of 
the tank to a pump with sufficient capacity to draw out the entire 
contents of the tank in half an hour. The outlet pipe from the pump 
discharged into the top of the tank, and the rapid circulation main¬ 
tained in the sewage in this manner during the experiment made 
sedimentation of solids practically impossible. Before the beginning 
of the test the sewage was pumped through the system for fifteen 
minutes in order to insure thorough mixing; then a sample was 
collected for chemical and bacterial analysis, and the calculated 
amount of disinfectant solution was slowly poured into the inflowing 
stream. The force of this stream entering the tank from a height of 
about six feet churned the sewage thoroughly and assured rapid and 
complete mixing. Samples were collected at the end of the second 
and fourth hours for determination of the total bacteria growing at 
20° C. and of the number of B. coli. The available chlorine employed 
76474 — irr 229—09 - 4 


48 


DISINFECTION OF SEWAGE. 


was ten parts per million. In the first series of tests the total amount 
of chloride of lime was added at first, but in the second series it was 
added in four portions at hourly intervals. The results are tabulated 
in Table 22. 


Table 22. —Disinfection of crude sewage with chloride of lime at Boston; series 1 and %. a 


Series.6 

Temper¬ 

ature. 

Number of bacteria per cubic 
centimeter. 

Initial. 

At end of 
2 hours. 

At end of 
4 hours. 


°F. 





44 

700,000 

4,900 

13,000 


46 

260,000 

220 

5,700 

1. 

SO 

750,000 

10 

100 


60 

600,000 

620 

80 


60 

200,000 

500 

100 


60 

270,000 

500 

100 

Average.. 

54 

460,000 

1,100 

3,400 


f 56 

600,000 

700 

190 


60 

460,000 

100 

60 

2. 

1 60 

360,000 

260 

40 


1 60 

460,000 

14,000 

340 


60 

280,000 

22,000 

200 


l 60 

540,000 

7,000 

130 

Average. 

59 

450,000 

7,500 

160 

Number of B. coli per cubic centimeter; average of both series. 

1 

35,000 

24 

15 


<* Available chlorine, 10 parts per million. 

b Series 1. Chlorine added at start. Series 2. Chlorine added in four equal hourly portions. 


The average reduction of total bacteria in the first series was 99.76 
per cent at the end of two hours, and, owing to multiplication of 
bacteria, 99.26 per cent at the end of four hours. In the second 
series the reduction in two hours was only 98.3 per cent, five parts 
per million of available chlorine having been added up to that time, 
and the addition of another five parts of chlorine brought the total 
reduction up to 99.96 per cent at the end of four hours. This indi¬ 
cates that there is a distinct advantage in adding the disinfectant in 
successive portions, but the increased efficiency in this case was not 
great, and it is doubtful whether it would generally be sufficient to 
compensate for the more complicated method of operation. 

The figures for B. coli have little individual significance, but the 
average values indicate a removal of B. coli of 99.93 per cent and 
99.97 per cent after two hours and four hours, respectively—values 
that are considerably better than those derived for the total bacteria. 
The results of the disinfection of crude sewage therefore agree with 
those obtained with the filter effluent. It is also apparent from these 
experiments that ten parts per million of available chlorine are ample 
to disinfect the crude sewage of Boston, and it is probable that even a 
less concentration would give satisfactory results. It is evident also 
that two hours is a sufficient contact period, little advantage being 
gained by making it longer. 























INVESTIGATIONS AT BOSTON. 


49 


In April, 1908, a third series of experiments with crude sewage 
was made to determine the minimum amount of chlorine and of con¬ 
tact time possible for satisfactory disinfection. These tests were 
essentially like those of the first series, except that the available 
chlorine was varied each day, so that on successive days it amounted 
to two, four, six, eight, and ten parts per million, respectively. On 
the sixth day it was started again at two parts. In this way the 
effect of chlorine concentration was studied with as little interference 
as possible from other variables, such as temperature and character 
of the sewage. A two-hour period of contact was given, and in ad¬ 
dition to the initial sample, in each test, samples were collected after 
half an hour, one hour, and two hours. The results of this series are 
shown in Table 23. 


Table 23. —Disinfection of crude sewage with chloride of lime at Boston; series 3. 


_ Available chlorine (in parts per 
million). 

Temper¬ 

ature. 

Number of bacteria per cubic centimeter. 

Initial. 

One-half 

hour. 

1 hour. 

2 hours. 


o 






52 

1,100,000 

90,000 

62,000 

130,000 


58 

1,600,000 

320,000 

470,000 

870,000 

o 

59 

1,200,000 

32,000 

49,000 

140,000 


53 

900,000 

47,000 

16,000 

42,000 


60 

1,900,000 

400,000 

450,000 

700,000 


64 

1,700,000 

370,000 

460,000 

1,000,000 


54 

2,100,000 

50,000 

41,000 

65,000 


55 

1,100,000 

5,000 

6,500 

4,400 

4 

. 59 

1,400,000 

30,000 

40,000 

85,000 


58 

950,000 

3,400 

1,100 

4,200 


60 

1,900,000 

12,000 

11,000 

23,000 


64 

3,500,000 

33,000 

26,000 

80,000 


56 

1,700,000 

3,600 

9,000 

1,700 


58 

1,700,000 

6,500 

600 

6,000 

ft 

60 

2,800,000 

17,000 

4,600 

4,400 


57 

1,700,000 

1,700 

8,400 

3,900 


60 

2,400,000 

1,800 

3,000 

1,700 


64 

3,300,000 

14,000 

11,000 

18,000 


55 

1,600,000 

570 

480 

600 


60 

1,500,000 

3,400 

160 

730 

o 

59 

2,000,000 

2,000 

1,100 

3,700 


59 

1,100,000 

340 

170 

750 


63 

2,600,000 

1,900 

2,700 

3,300 


65 

2,500,000 

6,300 

3,300 

9,500 


52 

1,600,000 

380 

300 

' 400 


57 

610,000 

200 

210 

200 

in 

58 

1,900,000 

1,100 

430 

1,600 


62 

1,100,000 

350 

800 

1,100 


- 64 

2,500,000 

500 

350 

850 


64 

3,100,000 

3,500 

2,300 

4,800 

Control...| 

56 

- 1,200,000 

1,500,000 

1,800,000 

2,100,000 


The results indicate that a large amount of chlorine is consumed 
by the sewage within two hours. Two parts per million are consumed 
within half an hour and ten parts per million are not sufficient to 
prevent the subsequent growth of bacteria. It has, however, already 
been pointed out that such subsequent growths have no significance 
and that the efficiency of the disinfection from the standpoint of the 
pathogenicity of the sewage is measured by the maximum reduction, 
independent of any subsequent increase. 























50 


DISINFECTION OF SEWAGE. 


The average initial and minimum figures and the per cent of bacteria 
removed for each concentration of chlorine are brought together in 
Table 24. 

Table 24. — Disinfection of crude sewage with chloride of lime at Boston; summary of 

results. 


Available 
chlorine (in 

Average number of bacteria per 
cubic centimeter. 

Bacteria 

parts per 
million). 

Average initial. 

Average 

minimum. 

removed. 

2 

1,400,000 

210,000 

Per cent. 

85 

4 

1,800,000 

23,000 

98.7 

6 

2,200,000 

6,000 

99.7 

8 

1,900,000 

1,200 

99.94 

10 

1,800,000 

700 

99.96 


These experiments were not sufficiently prolonged to furnish com¬ 
plete data on the process, and it is not at all unlikely that at another 
season of the year somewhat different results might have been ob¬ 
tained, though it has been shown in connection with the work on 
effluents that the temperature effect on the disinfection itself, other 
conditions remaining constant, is but slight. The bacterial and 
chemical composition of crude sewage, however, is affected by tem¬ 
perature to a much greater extent than is that of filter effluents. 
The amount of dissolved oxygen present is an important index to the 
character of the sewage or the effluent in reference to its effect on 
chlorine; absence of dissolved oxygen in particular indicates ready 
oxidizability, which in turn means the rapid absorption of chlorine. 
It is probably true that a given amount of chlorine acting for a given 
time will bring about a definite reduction in the total number of 
bacteria, regardless of the character of the treated liquid. Unless 
excessive amounts of chlorine are added, however, the time of con¬ 
tact is determined by the character and the amount of the organic 
matter in the sewage; consequently the more rapidly the chlorine is 
exhausted by this organic matter the shorter is the time of contact 
and the less is the bacterial reduction. The sewage experimented 
with in these tests represents about an average condition in regard to 
reducibility, and it contained about one-half the amount of dissolved 
oxygen that would be found during the winter months. Results 
fully as satisfactory as these could doubtless be obtained during six 
months of the year with even less available chlorine. During the 
summer months, when dissolved oxygen is not present, more chlorine 
than is here indicated would be found necessary. The Red Bank 
experiments on septic sewage make it probable that a concentration 
of ten parts per million of available chlorine may be taken as a maxi¬ 
mum amount during the summer. 

Satisfactory disinfection of crude Boston sewage can be accom¬ 
plished by adding chloride of lime in such amounts that the available 








DISINFECTION OF SEPTIC SEWAGE. 


51 


chlorine will amount to about five parts per million during six months 
of the year, and to between five and ten parts during the other six 
months, or an average amount during the year of seven or eight parts 
per million. The addition of the disinfectant in portions at intervals 
during the treatment yields results that are somewhat better than 
those obtained by adding the entire amount at once, but it is not 
probable that this advantage is commensurate with the additional 
complications involved. 

DISINFECTION OF SEPTIC SEWAGE AT RED BANK, N. J. 

The successful results obtained in the disinfection of trickling-filter 
effluents at Boston in 1906 warranted the extension of the experiments 
to include a practical demonstration; and that step was made possi¬ 
ble through, the cooperation of the New Jersey State Sewerage Com¬ 
mission. Red Bank, a town of about 6,500 inhabitants, situated on 
Navesink River in Monmouth County, was selected for the tests for 
two reasons: The works at Red Bank were so arranged that the 
experiment could be undertaken at a minimum expense for new con¬ 
struction; and, like many other communities in that section of the 
State, the town was discharging partly-purified sewage into waters that 
shortly reached important shellfish areas. A septic sewage of rather 
poor quality was to be treated. Work with an effluent of higher 
grade had been desired, but the novelty of treating a septic sewage 
and the immediate practical value of the results promised to make 
this experiment well worth undertaking. The conditions resembled 
those described on pages 12-13, where it was suggested that in certain 
localities thorough disinfection of crude or septic sewage will go far 
toward accomplishing all that is needful. At least the most necessary 
step is thus taken first, and it is a logical first step in the gradual 
improvement of sanitary and economic conditions along our water 
fronts. 

It was realized that more chloride of lime would be needed than had 
been used in previous experiments, but how much more was not 
known. The Boston results with crude sewage were not available at 
that time, and it was to be expected that, owing to the highly un¬ 
stable character of septic sewage, even more chloride of lime would 
be necessary than for crude sewage. The work was started in Octo¬ 
ber, 1906. During the fall the disinfection apparatus was left to the 
care of a local attendant, and a representative of the commission 
visited the works twice a week to collect samples for analysis; but 
mechanical deficiencies in the dosing device led to intermittent and 
irregular dosing, so that the results obtained were far from satisfac¬ 
tory, and considerable time was consumed before it was learned that 
more than 10 parts per million of available chlorine would be required. 
The work was discontinued when cold weather began, but it was 
resumed under more favorable circumstances in the summer of 1907. 
A laboratory in charge of Mr. Daniels was installed at the works, and 


52 


DISINFECTION OF SEWAGE. 


the entire experiment was then under careful observation, for samples 
could be taken and examined as frequently as might be desired. The 
results amply justified this course. The earlier work indicated the 
probable strength of chlorine necessary, and developed many defects 
and weaknesses in the dosing apparatus, all of which were remedied. 
The investigations that were conducted during the summer of 1907 
only are reported. 

The sewerage system of Red Bank is constructed on the separate 
plan, with special drains to carry off rain water. On account of the 
sandy nature of the soil and the poor construction of the sewers 
considerable storm water finds its way into the sewers. The average 
dry-weather flow of sewage is about 265,000 gallons a day. The 
following description of the disposal works will be more readily 
understood by referring to Plate I. After entering the works the 
sewage divides into two streams, passing through two grit chambers, 
each 5 by 8 feet in plan and 3 feet deep to the flow line. The flow 
may be entirely diverted through either tank to allow the other to 
be cleaned out. From these chambers the sewage flows to the septic 
tank, which is circular in plan, 43 feet in diameter, with a normal 
depth to the flow line of 8 feet 6 inches at the periphery and 5 feet 
at the center, the bottom being conical. Its capacity at that depth 
is about 82,000 gallons, or approximately an 8-hour dry-weather 
flow. For the purpose of these experiments, however, it was nec¬ 
essary to raise the flow line throughout the system 1 foot, a change 
that. increased the capacity of the tank to about 93,000 gallons. 
At the side of the tank opposite the entrance pipe the septic sewage 
passes out through a submerged 12-inch pipe i, which later divides 
into a Y. A branch then runs downward to the bottom of each of 
the filters, thence horizontally to a point beneath the center, thence 
upward into the filter, as shown in the sectional view. The so-called 
filters are two in number, and each is 12 feet in diameter. Tank a 
is 9 feet deep to the flow line, with a capacity of 7,600 gallons, and 
tank b is 8 feet 3 inches deep, with a capacity of 7,000 gallons. The 
tanks were originally filled with stone and brick over a false bottom 
and were practically strainers for the removal of suspended matter. 
Each filter is provided with a 12-inch overflow leading to a manhole 
outside the building. For these experiments the filtering material 
was entirely removed from both tanks, one of the outlets was closed, 
and a cross-connection was put in to conduct the sewage from the 
top of the first tank to the bottom of the second. By closing the 
inlet valve of the latter the entire flow was made to pass through 
the two tanks in series. Suitable baffles were provided, as indicated 
in the plan, so that the entire capacity of the tanks could be utilized 
to the best advantage. As thus arranged, each tank held about 
forty-five minutes’ flow, and the efficiency of the process was tested 
at the. end of forty-five minutes and ninety minutes. 


U. S. GEOLOGICAL SURVEY 


WATER-SUPPLY PAPER NO. 229 PLATE I 



Sterilizing 

tank. 


Septic tank 


Grit chambers 


Sterilizing 

tank 


- Superstructure 
-> Course ofsewage 


Present ' water line 


Norma / \ water line 


V777^07777P/77777Z7777/. 




^>>>>>>>>>>/ 


SECTION ON LINE X- 3 / 


PLAN OF SEWAGE-DISPOSAL WORKS AT RED BANK, N. J. 









































































































































































. 




























DISINFECTION OF SEPTIC SEWAGE. 


53 


Chloride of lime was used throughout the work. For the prepara¬ 
tion of the solution two hogsheads c and d of about 240 gallons 
capacity each were provided, one elevated over the other. In the 
upper hogshead the requisite amount of bleaching powder was mixed 
with water and allowed to settle over night. The next morning the 
clear supernatant liquid was drawn into the lower hogshead. The 
latter was connected directly with a constant-level tank e, controlled 
by a ball cock, on which a glass ball and special fittings were neces¬ 
sary to prevent corrosion. The constant-level tank was connected 
by a flexible rubber tube with the dosing tank/—a box 6 by 6 inches 
in plan and 8 inches deep—provided with a three-sixteenth-inch 
orifice in one side and suspended on one end of a 6-foot lever g, the 
other end of which was connected with a large float h in tank a. 
Tank a was provided at its outlet with a 1-foot weir, by which the 
water level was made to vary with the flow. An increase in the flow 
increased the elevation of sewage in the tank, thus raising float Ji 
and lowering the suspended box / at the other end of the lever. 
Since box/ was connected with the constant-level tank e, the effect 
of such change was virtually to increase the head over the orifice. 
In this manner the flow of the hypochlorite solution was automat¬ 
ically kept about proportional to the flow of sewage. The ratio was 
not exact, because the variation in flow with the head is not the 
same with a weir as with an orifice, but within the range of flows 
used the arrangement was quite satisfactory. By discharging the 
sewage from tank a through a submerged pipe instead of over a 
weir the ratio between the two flows could be kept the same at all 
heads. A suitable gage was erected in tank a, and the elevation of 
the sewage over the weir crest was recorded at intervals of two hours 
from 6 a. m. to 6 p. m., or more frequently during storms. These 
readings were converted into gallons, and the daily discharges of 
sewage were calculated from them. The night flow was approxi¬ 
mately determined by calculations based on two sets of hourly 
observations extending over twenty-four hours and on the relation 
of these sets of data to the general averages of day flows. When 
the weir gage was read, readings were also taken of a gage in the 
hogshead c, calibrated to read directly in gallons. By this means 
the hourly flows of disinfectant solution were known at all times 
and could be compared with the corresponding sewage flows. 

The solution of bleaching powder was run into the septic tank 
outlet pipe i, where it mixed with the main flow of sewage. The 
treated sewage then passed through the two disinfecting tanks a and b 
and was finally discharged into the river. The extraction of the 
hypochlorite was not complete in the one leaching given it. In 
mills where large quantities of it are extracted three successive 
lixiviations are customary in order to complete the extraction, but 
such practice was not feasible here, and the sludge remaining each 


54 


DISINFECTION OF SEWAGE. 


day in hogshead d was dumped into tank a, where it settled to the 
bottom and was slowly leached out by the sewage. In this way all 
the available chlorine was utilized. 

It was necessary to have some information about the chemical 
composition of the septic sewage under treatment and to make cer¬ 
tain chemical tests of the effluent. For this purpose samples of the 
septic sewage and of the disinfected effluent were collected daily and 
determinations of the oxygen consumed were made on them. These 
estimates gave a comparative indication of the strength of the 
sewage from day to day. Daily determinations of dissolved oxygen 
in the final effluent were also made, and the effluent was tested for 
free chlorine. The septic sewage never contained any dissolved oxy¬ 
gen, and under normal conditions the effluent would be in the same 
condition, but the application of free chlorine to sewage gives rise to 
chemical reactions, whereby free oxygen is formed. This is largely 
consumed in the oxidation of the organic matter, but nevertheless 
some free dissolved oxygen was always found in the effluent. The 
presence of this oxygen is of great value in the further self-purifica¬ 
tion of the sewage after reaching the river. Free chlorine was never 
found in the final effluent. Its presence would be highly injurious 
to fish life, and disinfection processes of this nature must always be 
so controlled that no injurious chemical can escape into the stream. 
In addition to the daily determinations just .mentioned, composite 
sterilized samples were preserved and sent to Boston regularly, 
where they were submitted to complete chemical analysis. Regular 
chemical analyses of the bleaching powder and daily assays of the 
disinfectant solution were made. 

The results of the work at Red Bank are given in the form of 
weekly averages in Table 25, and the relation between theTndividual 
and the average results are shown in Table 26. 


Table 25. —Disinfection of septic sewage with chloride of lime at Red Bank, N. J.; 

weekly averages. 


Week ending. 

Avail¬ 

able 

chlo¬ 

Total number of bacteria per cubic 
centimeter at 20° C. 

Number of B. coli per cubic cen¬ 
timeter. a 

rine in 
parts 
per 

million. 

Initial. 

At end of 45 
minutes. 

At end of 90 
minutes. 

Initial. 

At end of 45 
minutes. 

At end of 90 
minutes. 

1907. 

July 20. 

9.9 

800,000 

410 

460 

46,000 

4 

4 

27. 

10.6 

650,000 

800 

420 

80,000 

13 

11 

Aug. 3. 

11.5 

1,800,000 

550 

130 

40,000 

21 

5 

10. 

11.4 

856.000 

240 

140 

55,000 

14 

2 

17. 

13.0 

760,000 

2,100 

1,500 

70, U00 

30 

28 

24... 

7.3 

700,000 

45,000 

55,000 

70,000 

700 

600 

31. 

7.5 

1,200,000 

45,000 

26,000 

220,000 

16,000 

2,000 

Sept. 14.. 

11.8 

750,000 

13,000 

8,000 

300,000 

150 

140 

21. 

13.1 

750,000 

850 

800 

500,000 

270 

260 

28.. 

10.5 

700,000 

120 

88 

550,000 

80 

28 

Average &. 

11.5 

900,000 

2,300 

1,400 

205,000 

75 

60 


a Jackson bile media used. & Exclusive of period August 19-31; temperature 56° to 58° throughout. 



























DISINFECTION OF SEPTIC SEWAGE. 55 


Table 26 .—Relation between individual tests of bacterial removal and the average result 

at Red Bank, N. J. 


Per cent of total 
number of tests .a 

Total number of bacteria 
per cubic centimeter. 

Not less 
than— 

Less than — 

84 

100 

1,000 

5 

1,000 

2,000 

5 

2,000 

5,000 

6 

5,000 


Total, 100 

Average, 1,900 


a Total number of tests, 224. 


During the period August 19 to August 31, the available chlorine 
was reduced to about 7.5. parts per million. A removal of the total 
bacteria amounting to about 95 per cent and of B. coli amounting to 
94.3 per cent after three-quarters of an hour and 99 per cent after 
one and a half hours was obtained, but the individual results were 
very erratic, and it was apparent that an insufficient amount of 
chlorine was being added. The chlorine was therefore increased 
again to about 12 parts per million and kept there during the remain¬ 
der of the experiment. The averages in the last line of Table 25 do 
not include the results for the above-mentioned period. The average 
per cent removal of total bacteria was 99.7 at the end of 45 minutes, 
and the corresponding B. coli figure is 99.96. In 90 minutes the 
average per cent removals were 99.8 and 99.97, respectively. It is 
rather striking that such results can be obtained with 12 parts per 
million of available chlorine, while the results are poorer and very 
erratic with 7.5 parts. The explanation is probably found in the 
character of the sewage, which was very concentrated in its raw state 
and had passed through a septic tank that was working vigorously. 
A large amount of hydrogen sulphide was contained in the septic 
sewage. The characteristic odor of this substance was extremely 
noticeable in the vicinity of the works, and the addition of chlorine 
to the sewage produced a milkiness, due to the liberation of free sul¬ 
phur. Titration of the septic sewage in the cold with potassium 
permanganate, a substance less easily reduced than chlorine, indi¬ 
cated that enough hydrogen sulphide and other easily oxidizable 
substances were present to. reduce instantly about five or six parts 
per million of chlorine. Consequently the efficient results in this 
experiment were in reality obtained with about six parts of available 
chlorine and the poor ones with about three parts. 

The disinfection of septic sewage evidently requires so much 
chlorine that the expense will be considerable. It will probably be 
found to take twice as much chlorine for septic sewage as for the cor- 













56 


DISINFECTION OF SEWAGE. 


responding crude sewage. The combination of septic action and 
disinfection eliminates the suspended matter and the bacteria from 
the sewage and in many localities would constitute a very desirable 
system. In practice, however, the order of the processes should be 
reversed. The disinfection of the crude sewage can be done in a 
small tank of an hour’s capacity. The disinfectant should be so 
regulated that little or no chlorine would flow into the septic tank. 
Unless a considerable amount of chlorine should thus escape at one 
time there would be no objectionable effect on the action in the 
tank. There would be a great multiplication of bacteria in the 
tank, so that the number in the final effluent would probably be as 
great as in the raw sewage, and perhaps even greater. Nevertheless, 
the disinfection would be as effective on the pathogens as if it were 
applied as a final process. The subsequent development of sapro¬ 
phytes would have no sanitary significance and would doubtless be 
of real value in the subsequent self-purification of the organic matter 
after it had been discharged intp the stream. 

DISINFECTION OF TRICKLING-FILTER EFFLUENT AT BALTIMORE. 

Reference was made in the history of the experiments to the coop¬ 
erative arrangement between the United States Geological Survey 
and the Baltimore sewerage commission. In December, 1907, the 
sewerage commission undertook a study of disinfection methods at 
its Walbrook testing plant. The writer assisted in planning this 
work, and was consulted from time to time during its progress. The 
disinfection experiments were made a part of the routine work of the 
testing station, and they were carried out by the station staff. A 
complete review of this investigation, the most important and most 
comprehensive one on the subject ever conducted, will, it is hoped, 
be made by the sewerage commission in the near future. The pres¬ 
ent report has been made as brief as possible and includes only the 
work done before June 30, 1908. 

The Walbrook testing plant, in the extreme western section of 
Baltimore, was built for use in investigating certain problems in 
connection with the disposal works now being built at Back River, 
the main features of which—screening, sedimentation, treatment on 
trickling filters, and final sedimentation—had already been decided 
upon. Information was specially desired as to the probable bacterial 
efficiency of such a process and as to the necessity for further treat¬ 
ment of the effluent, because the statutes of the State of Maryland 
require rather high bacterial efficiency. As a detailed description of 
the testing plant has already been published,® it is sufficient to state 
that a small section of the city was sewered and that sewage amount- 


Eng. News, vol. 57, 1907, p. 235. 



DISINFECTION OF TRICKLING FILTER ' EFFLUENT. 57 

ing to about 50,000 gallons a day was brought to the plant and 
treated by a system comprising a grit chamber, a septic tank, trick¬ 
ling filters, and sedimentation tanks. Trickling filters of several 
depths and sizes of material are employed. The sewage is rather 
stronger than ordinary city sewage and is very fresh. The effluent 
used in the disinfection studies came from a filter 12 feet deep, built 
of crushed stone ranging in size from 0.5 inch to 1.5 inches. The rate 
of filtration fluctuated with the flow of sewage, but it averaged about 
3,000,000 gallons an acre a day. The effluent was conducted to a 
sedimentation basin designed to give a two hours’ flow. The disin¬ 
fectant employed throughout this investigation was chloride of lime, 
dissolved and added to the effluent in practically the same manner 
as at Boston, as the effluent left the filter drain. The disinfecting 
solution and the effluent mixed during the short flow through a pipe 
to the sedimentation basin. 

Monthly averages of the analyses of this effluent as it left the filter 
and after disinfection and sedimentation are given in Table 27. 

Table 27 .—Chemical analyses of tricMing-filter effluent at Baltimore before and after 
disinfection with chloride of lime and sedimentation: monthly averages. 

[Parts per million.] 

INITIAL. 




Suspended 

solids. 

Organic matter. 

Nitrogen as— 

Oxy¬ 


Month. 

Total. 

Loss 

on 

igni¬ 

tion. 

Total. 

Solids. 

Free 

ammo¬ 

nia. 

Ni¬ 

trites. 

Ni¬ 

trates. 

gen 

con¬ 

sumed.® 

.Tarmarv 

1908. 

56 

35 

8.3 

5.9 

7.4 

0.48 

• 17 

16 

February. 

38 

25 

8.0 

6.8 

6.8 

.13 

17 

17 

March. 

53 

31 

8.9 

7.5 

5.5 

.30 

19 

16 

April. 

42 

28 

10.0 

9.0 

7.5 

.08 

21 

18 

Jimp. _ _ 

33 

21 

7.0 

6.5 

4.0 

.40 

20 

13 






Average. 


45 

28 

8.4 

7.1 

6.2 

.28 

19 

16 







FINAL. 


1908. 

January. 

38 

26 

8.5 

6.2 

7.2 

0.49 

16 

15 

February. 

25 

17 

6.8 

5.8 

6.8 

.12 

17 

17 

March. 

26 

17 

7.9 

7.1 

5.5 

.19 

20 

15 

April... 

34 

20 

9.6 

7.6 

7.9 

.08 

22 

18 

June. 

21 

16 

7.0 

6.5 

4.0 

.25 

20 

13 

Average. 

29 

19 

8.0 

6.6 

6.3 

.23 

19 

16 


a Thirty-minute boiling method. 


Initial and final bacteriological samples were collected as usual. 
Bacterial counts were made three times a day for five days each week 
and twice on Saturday. B. coli determinations were made twice 
daily. The results are shown in Tables 28, 29, and 30. Table 28 










































58 


DISINFECTION OF SEWAGE 


gives the average weekly results; Table 29 shows the relation between 
the removal of bacteria in individual samples and the average 
removal; Table 30 indicates the relation between the number of 
bacteria in the individual samples and the average number. 


Table 28.— Disinfection of trickling-filter effluent with chloride of lime at Baltimore; 

weekly averages. 


Weekending— 

Temper¬ 

ature. 

Available 
chlorine 
(parts per 
million). 

Total number 
of bacteria per 
cubic centimeter 
at 20° C. 

Total number 
of bacteria per 
cubic centi¬ 
meter at 37° C. 

Number of 
acid formers 
per cubic cen¬ 
timeter at 
37° C. 

Number of 
B. coli per 
cubic centi¬ 
meter. “ 



Initial. 

Final. 

Initial. 

Final. 

Initial. 

Final. 

Initial. 

Final. 

Initial. 

Final. 

1908. 

° F. 










/ 

Jan. 4. 

50 

3.5 


170,000 

1,600 

70,000 

600 

8,000 

107 

3,800 

59 

11. 

45 

2.0 


90,000 

(1500 

5,500 

200 

3,000 

58 

1,700 

380 

18. 

48 

1.9 


120,000 

3,300 

3,100 

250 

2,200 

165 

3 , 400 

240 

25. 

47 

2.0 


160,000 

1,900 

2,000 

95 

900 

8 

1,300 

45 

Feb. 1. 

40 

2.6 

1.3 

120,000 

2,200 

1,900 

73 

950 

4 

2,000 

490 

8. 

43 

2.5 

1.1 

70,000 

4,100 

1,000 

70 

400 

4 

600 

470 

15. 

46 

0. 94 

0.1 

80,00C 

11,000 

1,500 

400 

700 

90 

440 

330 

22. 

45 

1.4 

0.4 

65,000 

20,000 

4,000 

440 

3,000 

180 

750 

70 

29. 

45 

1.5 

0.4 

100,000 

4,600 

3,000 

100 

1,100 

17 

2,400 

210 

Mar. 7. 

46 

2.0 

0.6 

80,000 

1,000 

1,400 

80 

900 

2 

1,600 

150 

14. 

47 

1.7 

0. 4 

80,000 

1,700 

1,000 

60 

500 

3 

1,400 

180 

21. 

49 

2.0 

0. 4 

100,000 

1,300 

1,100 

80 

600 

15 

2,600 

180 

28. 

52 

2.1 

0.3 

150,000 

1,700 

2,400 

160 

1,500 

3 

'800 

210 

Apr. 4. 

50 

1. 7 

0.4 

120,000 

1,800 

4,100 

120 

2,100 

10 

2,100 

220 

11. 

55 

2.6 

1.1 

140,000 

1,100 

2,800 

70 

1,900 

2 

3,700 

39 

18. 

54 

2.2 

0.6 

120,000 

2,500 

13,000 

110 

3,600 

25 

5 , 500 

55 

25. 

56 

2.6 

0.3 

120,000 

2,100 

18,000 

350 

7,000 

110 

2,200 

200 

May 2. 

58 

3.1 


130,000 

1,200 

3,500 

no 

? 400 

3 



9. 

56 

2.8 

1.0 

160,000 

2,300 

2,200 

70 

lj 600 

9 

1,800 

43 

J une 13. 

66 

2.3 

0.4 

120,000 

17,000 

21,000 

3,600 

9,500 

b 730 

600 

100 

20. 

67 

2.3 

0.8 

115,000 

1,300 

1,800 

440 

300 

18 

3,000 

300 

27. 

67 

1.9 

0.1 

190,000 

3,700 

2,900 

1,100 

430 

0 

'800 

400 

Average. 

51 

2.2 

0.6 

120,000 

4,300 

7,500 

390 

2,400 

70 

2,000 

200 


a Jackson bile media used. b High average due to one count of 8,000. 


Table 29.— Relation between individual tests of bacterial removal and the average result 

at Baltimore. 


Per cent of total 
tests.o 

Per cent of bacterial 
removal. 

Not less 
than — 

Less than— 

69 

98 

100 

14 

96 

98 

6 

94 

96 

0 

92 

94 

2 

90 

92 

2 

85 

90 

4 

75 

85 

1 

65 

75 

1 

40 

65 

1 


40 

Total, 100. 

Average, 95.5. 


“Total number of tests, 255. 













































































DISINFECTION OF TRICKLING FILTER EFFLUENT. 


59 


Table 30. —Relation between individual bacterial counts and the average result at 

Baltimore. 


Per cent of total 
tests.® 

Number of bacteria per 
cubic centimeter. 

Not less 

1 than— 

Less than — 

36 

100 

1,000 

35 

1,000 

2,000 

8 

2,000 

3,000 

3 

3,000 

4,000 

4 

4,000 

5,000 

2 

5,000 

6,000 

2 

6,000 

8,000 

3 

8,000 

10,000 

7 

10,000 


Total, 100. 

.'-•"erage, 4,300. 


a Total number of tests, 255. 


The average of the individual bacterial removals is 95.5 per cent. 
The per cent removal calculated from the average numbers in Table 
28 is 96.6 per cent. The average efficiency, as in the Boston experi¬ 
ments, may be expressed in the latter form with sufficient accuracy, 
and this shorter method is employed in the other bacterial compu¬ 
tations, by which the efficiencies in Table 31 were obtained. 


Table 31. —Average bacterial removal during the disinfection of trickling-filter effluent 

at Baltimore. 


Per cent. 


Total bacteria at 20° C. 96. 6 

Total bacteria at 37° C.. 94. 9 

Acid formers at 37° C.. 97. 0 

B. coli a . 90. 0 


The average results agree practically with those obtained at 
Boston. The trickling-filter effluent at Baltimore was of better 
quality than that at Boston and less chlorine was used. The aver¬ 
age amount of available chlorine was 2.2 parts per million, as com¬ 
pared wdth 3.4 parts at Boston. Though the average reduction of 
total bacteria at both places was practically the same, the variations 
were greater at Baltimore, and they indicate that a somewhat greater 
amount of chlorine would materially improve the results. It is 
probable that three parts per million of available chlorine are best 
for treating this effluent. The B. coli results are not in harmony 
with the rest of the figures. In all the work at other places the 
removal of B. coli has been without exception better than the re¬ 
moval of the total organisms. Soon after the beginning of the 
Baltimore studies it was noticed that the removal of B. coli, as indi¬ 
cated by the bile test, was far less perfect than the general results, 
but efforts to discover the cause of the discrepancy were only partly 


Jackson bile media used. 















60 


DISINFECTION OF SEWAGE. 


successful. Complete identification of B. coli was carried out for a 
period, and the results seem to indicate that many of the positive 
tests after disinfection are due to the presence of an organism other 
than B. coli , which fermented the bile medium. Similar atypical 
results were obtained in the initial samples, but not in such large 
proportion. The evidence of other work under the direction of the 
author indicates that in these experiments the removal of B. coli is 
better shown by the removal of acid-forming bacteria than by the 
bile test. The results as a whole demonstrate the entire feasibility 
of the process and the possibility of obtaining practical disinfection 
at a reasonable cost. 

COMPARATIVE GERMICIDAL EFFICIENCIES OF CHLORINE AND 
SOME OF ITS COMPOUNDS. 

Only a small amount of accurate data was at hand for the com¬ 
parison of the germicidal efficiency of chlorine in its various forms. 
Accordingly, three series of comparative tests were made upon 
trickling-filter effluents for the purpose of obtaining such informa¬ 
tion. The results of these studies are collated in Table 32, each 
figure of which is the average of from three to twelve tests reduced 
to a uniform basis of one million initial bacteria per cubic centimeter. 


Table 32. — Relative germicidal properties of chlorine and some of its compounds. 
[All numbers converted to a uniform basis of 1,000,000 initial bacteria per cubic centimeter.] 


Series. 

Source of chlorine. 

Avail¬ 
able 
chlorine 
(in parts 
per 

mihion). 

Total number of remaining bac¬ 
teria per cubic centimeter. 

At end 
of 30 
minutes. 

At end of 

1 hour. 

At end of 

2 hours. 


Free chlorine. 

3 

650 

OQO 

qo n 


Sodium hypochlorite. 

3 

500 

410 

800 000 

oyu 

97n 

60 \J 

93n 

I 

Potassium hypochlorite. 

3 

u 

9nn 

ZoU 

oon 


Potassium chlorate. 

3 

6\J\) 

nnn nnn 

60 U 
i nnn nnn 


Potassium perchlorate. 

3 

750000 

yuu, uuu 

i 4nn nnn 

1 ,uuu, uuu 
i onn nnn 


(Free chlorine. 

2 

17 000 

A, 1UU, UUU 

1 3 nnn 

A ,OUU, uuu 

1 7 nnn 

II 

i Sodium hypochlorite «___ 

2 

15^000 

4 nnn 

uuu 

nnn 

1 /, uuu 
6,000 
q Ann 


1_do.6. 

2 

u, uuu 

9 inn 


[Free chlorine. 

2 

iq nnn 

6 , 1UU 

i ft nnn 

o, €UU 
oq nnn 


Sodium hydroxide and chlorine «... . 

2 

23,000 
99 nnn 

io,uuu 
14,000 

60 ,UUU 
iq nnn 

III 

1 —do.d. 

2 

±y,uuu 


_do.«. 

2 

66 , UUU 

93 nnn 

12,000 
q nnn 

18,000 

10,000 


1—do./. 

2 

60 , UUU 

iq nnn 

y, uuu 

7 *nn 

— — - 



uuu 

l, OUU 

8,000 


« Electrolytic. 

6 From bleaching powder. 

c Chlorine added thirty minutes after the hydroxide. 

d Chlorine added twenty minutes after the hydroxide. 

« Chlorine added ten minutes after the hydroxide. 

/ Chlorine added with hydroxide. 

In series I comparison is made of free gaseous chlorine generated 
electrolytically, sodium hypochlorite, and potassium hypochlorite. 
Potassium chlorate and potassium perchlorate are also included, 
because they are formed in many direct electrolytic processes. The 
available chlorine of the two latter compounds is taken in an elec- 































COMPARATIVE GERMICIDAL EFFICIENCIES. 


61 


trolytic equivalent sense, and would be better expressed by the term 
“oxidizing power.” Three parts per million of available chlorine 
w T ere used in all the tests of series I. 

In series II comparison is made of free gaseous chlorine, potassium 
hypochlorite prepared from bleaching powder, and the same com¬ 
pound electrolytically prepared by the recombination of the products 
of the electrolytic cell. In this and in the next series two parts per 
million of available chlorine were used, because it was found that 
comparisons are more readily made where the disinfection is not so 
nearly complete. 

In series III free chlorine was again used and was compared with 
the electrolytic hypochlorite. In this set of experiments, however, 
the hypochlorite was made in the sewage by adding separately the 
chlorine water and the sodium hydroxide. In three sets of tests 
the addition of the hydroxide preceded that of the chlorine by ten, 
twenty, and thirty minutes, respectively. The object of this pro¬ 
cedure is to determine whether the caustic soda and the gaseous 
chlorine possess separately better penetrating powers than the hypo¬ 
chlorite does. If such were the case, formation of the hypochlorite 
within the solid particles of the sewage might result in more com¬ 
plete disinfection than could otherwise be obtained. 

The results indicate plainly that hypochlorites are the most effi¬ 
cient germicides. Gaseous chlorine is almost as good, but in each 
series the free chlorine is somewhat inferior to the hypochlorite. 
Chlorates and perchlorates have almost no value in disinfection. 
The formation of these compounds in the electrolytic cell is, there¬ 
fore, a total waste of energy, and should be prevented as far as possi¬ 
ble. Production of these compounds explains in large measure the 
inefficiency of hypochlorite cells. Hypochlorites made electrolytic- 
ally are slightly inferior to the market product, but this difference 
would probably be inappreciable in large-scale tests, where the con¬ 
ditions under which the hypochlorites are prepared are more nearly 
those of commercial practice. Hypochlorites of different bases evi¬ 
dently have the same value; the results obtained with the sodium 
and the potassium salts are practically identical and are similar to 
those obtained in practice with the calcium salt. The claim that the 
magnesium compound is more efficient than the calcium compound 
was not investigated. Hypochlorites can be made advantageously 
by mixing dilute solutions of free chlorine and sodium hydroxide, the 
products of the electrolytic cell, but the mixing should take place 
either in the sewage or in cold dilute solution, as otherwise chlorates 
and perchlorates will be formed; if the hydroxide is added a short 
time before the chlorine, it is removed from the solution, leaving free 
chlorine uncombined and leading to a more rapid exhaustion of the 
available chlorine. 


62 


DISINFECTION OF SEWAGE. 


These tests have an interesting bearing on the question of the ex¬ 
haustion of the chlorine. It appears that the chlorine attacks the 
organic matter and the bacteria simultaneously, but that its effect on 
the former is a direct function of its concentration, while its germi¬ 
cidal effect does not bear such exact relation. If these are the true 
conditions, the successive addition of small portions of chlorine, or 
what amounts to the same thing, the addition of a substance that 
yields chlorine slowly, prevents the rapid reduction of the chlorine 
by the organic matter and prolongs the time of contact with the bac¬ 
teria. In series III, Table 33, more or less of the hydrate was con¬ 
sumed in saponification before the chlorine was added, the amount 
so removed depending on the time that elapsed between the addition 
of the hydroxide and the addition of the chlorine. The first set of 
series III, therefore, represents the action of free chlorine, and the 
last set the action of hypochlorite, with the intermediate tests repre¬ 
senting different proportions of the two substances. The progressive 
nature of the final results indicates strongly the necessity of having 
the chlorine combined in such form that its action on organic matter 
is retarded. The same effect is shown in the work on crude sewage 
described on pages 37-51. In the experiments reported in Table 22 
decidedly better results were obtained by adding the hypochlorite 
in four equal portions at hourly intervals than by adding the entire 
amount at once. In practice the effluent to be treated should never 
be acid in reaction; it is probable that the addition of lime would 
still further decrease the rate of decomposition of the hypochlorite 
and increase its bactericidal efficiency. 

EFFECT OF CALCIUM HYPOCHLORITE ON COLON AND TYPHOID 

BACILLI. 

The colon bacillus has been employed in this and in other work as 
a convenient test organism with which to measure the efficiency of 
the disinfection process. For obvious reasons experiments on a large 
scale with the typhoid bacillus are out of the question. Consequently, 
a comparison was made between the relative resistance of the typhoid 
and the colon bacilli under controllable conditions, with the idea that 
in practice the effect of the disinfection process on the former could 
be measured by its effect on the latter. Emulsions of the two organ¬ 
isms in tap water were treated with hypochlorite solution, and the 
parallel tests with the two species were made at the same time and 
were kept as nearly alike as possible. The number of bacteria per 
cubic centimeter was determined at the end of twenty, forty, and 
sixty minutes, and two, four, and eighteen hours, and twelve tests 
of each kind were made in the same manner. The available chlorine 
ranged from 3.5 to 6 parts per million, averaging 5 parts. The 
results of the individual tests varied greatly from day to day, because, 


PRACTICAL APPLICATIONS AND COSTS. 


63 


fio doubt, of difference in the character of the growths and in the 
amounts of organic matter introduced with the organisms. Yet the 
average figures obtained from these twelve sets of tests probably give 
a fair basis for estimating the comparative resistance of the two 
organisms to the disinfectant. The per cent removal has been cal¬ 
culated and the average results are presented in Table 33. 


Table 33 —Comparative resistance to calcium hypochlorite of B. typhi ami B. coli in 

aqueous emulsion.a 


Tests made at end of— 

Removal of bacteria 
(percent). 

B. typhi. 

B. coli. 

20 minutes. 

90.5 

98.2 

99.45 

99.60 

99.92 

99.99+ 

92.0 

98.0 

99.53 

99.70 

99.96 

99.99+ 

40 minutes. 


2 hours. 

4 hours. 

18 hours. 



a Average available chlorine, 5.0 parts per million. 


The slight differences shown by the experiments on the two organ¬ 
isms may be attributed to experimental variations. The work is 
not conclusive because other strains of organisms might have yielded 
different results, but it indicates in a general way that B. coli may 
reasonably be regarded as test organisms in disinfection work and 
that the process may be expected to destroy typhoid organisms 
present at least as thoroughly. 

It is interesting also to note that the per cent removal of B. coli 
after four hours is about the same as that recorded in the experi¬ 
mental disinfection of crude sewage; namely, 99.96 per cent as com¬ 
pared with 99.93 per cent. 

PRACTICAL APPLICATIONS AND COSTS OF DISINFEC¬ 
TION. 

The experiments that have been described were sufficiently pro¬ 
longed and varied in their scope to justify the application of the 
chloride of lime treatment to practical disinfection on a large scale. 
It is not possible, however, to draw general conclusions regarding the 
amount of chlorine necessary for the disinfection of effluents, much 
less of crude sewage, because the dose is determined largely by the 
character of the organic and reducing -matters contained in the sewage 
or the effluent. But the investigations have shown what may be 
accomplished under several conditions and they have established 
certain probable maximum amounts of chlorine for particular classes 
of sewage and effluents. They have demonstrated, for example, that 
trickling-filter effluents similar to those tested may be satisfactorily 
76474— irk 229—09-5 




















64 


DISINFECTION OF SEWAGE. 


disinfected with three or four parts per million of available chlorine, 
and that such quantity will usually be sufficient to effect the removal 
of 95 per cent or more of the total bacteria in the effluent. If the 
effluent should contain only 25 per cent of the original sewage bac¬ 
teria, the whole purification process would result in a removal of 
98.8 per cent of the total number of bacteria. The per cent removal 
of B. coli and of typhoid organisms will be at least as high if not 
higher. Crude Boston sewage can be disinfected to about the same 
extent with from four to six parts per million of available chlorine. 
It may reasonably be inferred, therefore, that five parts per million 
of chlorine represent the maximum concentration that would ever 
be necessary for the disinfection of a fairly stable effluent, since no 
such effluent would contain as much oxidizable organic matter as 
the crude sewage that is treated. The septic sewage of Red Bank 
probably represents a maximum condition for crude sewage, because 
its high content of hydrogen sulphide and other oxidizable matters 
would probably never be exceeded in crude sewage, or at least in 
sewage from American cities. The disinfection of crude sewage, 
therefore, would require four to twelve parts per million, of available 
chlorine, depending on the character of the sewage and its content 
of oxidizable matters. It will probably be undesirable to treat septic 
sewage, but if such sewage should be treated, from ten to fifteen parts 
of chlorine, or perhaps more, would be necessary. The extreme 
variability of the composition of septic sewage makes it almost 
impossible to fix a maximum limit for the amount of hypochlorite. 
The amount of disinfectant required in any plant would necessarily 
be determined before final adoption of the plan, but these estimates 
will serve as useful guides, and the limits assigned include the maj ority 
of probable conditions. 

No data have been obtained with effluents of higher degree of purity 
than those from trickling filters. The general conclusions from the 
work have been that the disinfectant action is a function of the chlo¬ 
rine concentration and of the time of contact, and that the time is 
determined largely by the rate at which the chlorine is consumed by 
the organic matter. Effluents of a better grade would probably 
require the same amount of chlorine, if the disinfection were to be 
accomplished within two hours. In the effluents with which experi¬ 
ments were made, the chlorine was practically consumed in two 
hours, but in a purer effluent chlorine would probably be left. This 
indicates that somewhat smaller quantities could be used with longer 
storage periods. A point is soon reached, however, beyond which 
the cost of storage is greater than the saving in disinfectant. Effluents 
of high degree of purity could probably be disinfected with one part 
of available chlorine, but a contact period of at least five hours would 
be required for satisfactory removal of the bacteria. One part is 


PRACTICAL APPLICATIONS AND COSTS. 


65 


probably the minimum amount of chlorine that can be used, as the 
necessary time of contact increases very rapidly with decreasing 
concentration of chlorine. 

One of the most important practical points that has been devel¬ 
oped is the relation between organic matter, time of contact, and 
amount of chlorine necessary. Obviously a definite concentration of 
chlorine acting for a given time will bring about a definite result 
independent of the nature of the solution. If, however, chlorine is 
being consumed by the solution itself a greater amount will have to 
added at first in order to maintain the same average concentration. 
On the other hand, longer periods of contact with lower chlorine con¬ 
centration are possible with the better grades of effluent. The experi¬ 
ments with various kinds of sewage make it possible to formulate a 
crude rule, which may be stated as follows: The product of the 
initial concentration multiplied by the time in hours required for 
complete reduction of the available chlorine should be about five for 
satisfactory disinfection, except that the contact period must not be 
less than half an hour in any case. 

It is possible to fix within narrow limits the cost of chlorine disin¬ 
fection. The cost is determined chiefly by the concentration of 
chlorine necessary and the related factor of contact period, and 
secondarily by the size of the plant.' The unit used in the following 
summary of costs is a plant with a daily flow of 5,000,000 gallons of 
sewage, such a plant being the smallest that would require the entire 
time of an attendant, a part of whose salary may legitimately be 
charged to disinfection. On larger works the labor costs would 
increase proportionately, and on smaller works arrangements would 
naturally be made by which the plant could receive proper attention 
in connection with the regular work of the sewer or street depart¬ 
ment. The price of bleaching powder has been estimated at $24 a 
ton delivered at the plant; it is quoted at from $22 to $25, and a 
price as low as $20 is obtained on large orders by certain paper mills. 
The price taken is, therefore, sufficiently high to cover the cost of the 
moderate-sized shipments that would be required for a 5,000,000- 
gallon plant. A 50,000,000-gallon plant could obtain its bleaching 
powder for at least 10 per cent less. This bleaching powder would 
contain over 35 per cent available chlorine, but in order to allow for 
waste 33 per cent has been taken as the average figure. Labor is 
computed at $2 for an eight-hour day. Two hours a day—an ample 
allowance—are reckoned for the care of a 5,000,000-gallon disinfect¬ 
ing plant using 5 parts or less of chlorine. For over 5 parts the time 
would increase proportionately. Interest, depreciation, and other 
fixed charges are computed, as 6 per cent of the cost of the additional 
works made necessary by the disinfection treatment; as the con- 


66 


DISINFECTION OF SEWAGE. 


struction is chiefly masonry, but little depreciation need be allowed. 
In certain projects sedimentation tanks would already be available, 
so that the application of disinfection would not require the con¬ 
struction of storage tanks. For this reason the item storage tanks 
has been separated from the other fixed charges that include 6 per 
cent of the cost of mixing tanks and storage tanks for the solution, 
pumps, piping and connections, and suitable housing. In handling 
the chemicals for a plant requiring 3 parts per million or more of 
chlorine, some form of power mixer would be economical, and that 
item has been estimated. In small plants the mixture of bleaching 
powder and water would be settled and the solution decanted, but in 
the larger works the whole mixture would be used, and it would 
require constant stirring. The increased power item would be more 
than counterbalanced by the saving in tank construction. Table 34 
contains a summary of the cost estimates for chlorine concentrations 
from 1 to 15 parts per million. 

Table 34. — Estimates of the cost of maintenance and operation of a plant for disinfecting 
sewage or effluent with chloride of lime, based on a capacity of 5,000,000 gallons a day. 


Avail¬ 
able 
chlorine 
in parts 
per 

million). 

Time of 
contact 
(in 

hours). 

Cost per million gallons. 

Storage 

tanks. 

Other 

fixed 

charges. 

Bleach¬ 

ing 

powder. 

Labor. 

Power. 

Total. 

1 

5.0 

SO. 10 * 

SO. 02 

SO. 30 

SO. 10 


SO. 52 

2 

2.5 

.05 

.04 

.60 

.10 


.79 

3 

1.6 

.04 

.05 

.90 

.10 

SO. 02 

1.11 

4 

1.2 

.03 

.07 

1.20 

.10 

.02 

1.42 

5 

.8 

.03 

.08 

1.50 

.10 

.03 

1. 74 

10 

.5 

.02 

.16 

3.00 

.15 

.06 

3.39 

15 

.5 

.02 

.24 

4.50 

.20 

.09 

5.05 


The estimates made for a 5,000,000-million gallon unit can be 
safely applied to larger works, and they are applicable within reason¬ 
able limits to small works that are properly managed. The cost of 
the tanks and of covering them would become proportionately 
cheaper with increased size, but this saving would be offset by the 
advisability of better construction and architectural embellishment. 
The saving due to decreased price of bleaching powder for a 
50,000,000-gallon plant, using 3 parts per million of chlorine, would 
be about $4.50 a day, which would approximately pay for the 
additional chemical and bacteriological control required by a plant 
of such size. On very large works labor items would not increase 
proportionately, because most of the work would be done by 
machinery. 

In an earlier paper® the opinion was expressed that the expense 
might be considerably reduced and efficiency increased by using 

a Phelps, E. B., and Carpenter, W. T., The sterilization of sewage filter effluents: Tech. Quart., vol. 19, 
1906, p. 382; Contr. from Sanitary Research Lab., vol. 4, 1908. 
















PRACTICAL APPLICATIONS AND COSTS. 


67 


electrolytic chlorine produced at the disposal works instead of 
bleaching powder. The relation of electrolytic processes to sewage 
disinfection is still unsettled, but a study of the present possibilities 
of such processes and experimental work with the McDonald cell 
and with small cells of special design have indicated that the margin 
of cost between the alternative methods is so slight that it hardly 
justifies the additional effort and the uncertainty involved in the 
establishment of an electrolytic plant. A brief discussion of recent 
developments in this field will show the reasons for this conclusion. 
Two general types of electrolytic process are available, in both of 
which the electric current is passed through a solution of sodium 
chloride, chlorine being liberated at one electrode and caustic soda 
at the other. In one type these products are allowed to recombine, 
for min g sodium hypochlorite and certain other compounds. In the 
other type the products are removed from the cell as quickly as 
possible, the aim being to prevent their recombination. Numerous 
processes of the first kind have been developed, of which the Hermite 
and Woolfe processes have already. been mentioned. The com¬ 
mercial preparation, called “Chloros,” is made in this way. The 
most recent, and probably the most improved, cell of this type has 
recently been described by Digby and Shenton.® The reaction by 
which the hypochlorite is produced from chlorine and caustic soda 
in cold dilute solution is: 

(1) 2 NaOH + 2C1 = NaOCl + NaCl+H 2 0. 

In the paper just cited Digby proposes the reaction, 

(2) NaOH + Cl = NaOCl + H. 

He bases his view on the observation that the electro-chemical 
efficiency of the cell is over 50 per cent. Aside from the obvious 
impossibility that a reaction can produce at one and the same time 
nascent hydrogen and a strong oxidizing agent, it is apparent that 
reaction (1), if it were carried out completely, would yield a product 
containing not 50, but 100 per cent of the available chlorine initially 
present. The conception that this reaction represents a loss of half 
the available chlorine is due apparently to a mistaken idea of the 
term available chlorine, which, as has been explained on page 18, is 
really a misnomer. The fact is that the oxidizing power, or the 
available chlorine as ordinarily determined, of the products of reaction 
(1) is equivalent to twice the chlorine of the hypochlorite, or to the 
total chlorine present. There is, therefore, no apparent basis for the 
reaction proposed by Digby, which would yield twice as much avail¬ 
able chlorine as the amount allowed by the law of electro-chemical 
equivalents. The reactions of equation (1) are complete only in cold 

a Digby, W. P., and Shenton, H. C. H., Surveyor, vol. 30, 1906, p. 663. 



68 


DISINFECTION OF SEWAGE. 


dilute solutions. If the solution is hot or if it is concentrated chlorates 
and perchlorates are produced simultaneously. It is for this reason 
that the disinfectant value of these two sets of compounds was deter¬ 
mined in an earlier part of the present investigation. It was found 
that they possess practically no disinfecting power and that their 
production in the cell represents a loss of energy. Economy in elec¬ 
tric current demands strong salt solutions and high current densities 
with consequent heating of the electrolyte. Electrical efficiency is, 
therefore, opposed to chemical efficiency, and the problem in design¬ 
ing cells of this type is to balance the two efficiencies in the most 
economical manner. The electro-chemical equivalent of a current of 
one ampere is 1.32 grams an hour of chlorine, and this equivalent is 
not modified by the voltage. However, as the total energy employed 
in a process determines its cost, it is necessary to consider voltage as 
well as current in discussing electrolysis^ In any electrolytic reaction 
there is a definite minimum voltage required for carrying out the 
reaction, and this can be computed from thermal considerations as 
follows: The complete reaction in the cell before the recombination 
of the hydroxide and the chlorine may be written, 

NaCl + H 2 0 = NaOH + H + Cl. 

Substituting the heats of formation of these compounds gives 
964 + 684 = 1118+ x, whence 
x = 530 calories (K). 

Here x is the heat required by the reaction, expressed in calories (K) 
per gram-equivalent of the reacting substances, and its value is 530 
calories. One gram-equivalent .of substance is transformed with the 
passage of 96,540 coulombs of electricity, and one coulomb trans¬ 
ferred under a difference of potential of one volt has an energy 
equivalent to 0.00241 calories. Therefore, one electro-chemical 
equivalent of current at a difference of potential of one volt has a 
heat value of 96,540 multiplied by 0.00241, or 232.7 calories. Con¬ 
sequently the voltage required to effect the desired reaction, which 
calls for a heat absorption of 530 calories per equivalent, is 530 
divided by 232.7, or 2.28 volts. This is the minimum voltage with 
which the reaction can take place, and efficiency calculations to an 
energy basis must be referred to this voltage. At a difference of 
potential of 2.28 volts, one kilowatt gives 439 amperes, so that an 
output of 579 grams of chlorine per kilowatt-hour represents a process 
of 100 per cent efficiency on both a current and an energy basis. In 
practice a current at a tension of at least 4.5 volts is usually found 
necessary, even with strong solutions of salt and with electrodes 
placed as near together as possible. This factor alone reduces the 
energy efficiency to 55 per cent with perfect current efficiency. The 


PRACTICAL APPLICATIONS AND COSTS. 


69 


current efficiency depends especially on the design of the cell. If 
there are no complicating secondary reactions during the recombina¬ 
tion of the products it approaches 100 per cent very closely, and this 
is also the case in the most improved design of chlorine cell in which 
recombination does not take place. It is clear that the production of 
hypochlorite in one operation within the cell is not economical, and a 
review of the available information and a laboratory study of various 
hypochlorite cells have led to the conclusion that cells of that type 
can not be expected to yield much more than one-half the available 
chlorine that can be obtained from the same electric current by means 
of direct chlorine cells. 

Direct chlorine cells have been developed to a high degree of effi¬ 
ciency, and this is the type of cell which has been considered in the 
present studies of sewage disinfection. The McDonald cell, which 
was used to some extent in this work, is giving in a regular installa¬ 
tion in a large paper mill a current efficiency of over 80 per cent at 
4.5 volts, making the total energy efficiency over 43 per cent. The 
most recent development is the Townsend cell, for which current 
efficiencies exceeding 98 per cent are claimed at a tension of 5 volts 
or more. The special feature of the Townsend cell is an arrangement 
by means of which the caustic liquor drops into a bath of oil as soon 
as it is formed, a step that prevents recombination. In spite of the 
high efficiency of such cells, it is not practicable to employ them in 
sewage work on account of the small margin between the market cost 
of chlorine and the cost of its manufacture electrolytically. This con¬ 
dition is due to the fact that the demand for caustic soda is so much 
greater than that for chlorine that the chlorine is to a certain extent 
a by-product and can be made into bleaching powder and sold at low 
cost. On the other hand, the manufacture of small amounts of 
caustic liquor at the disposal works does not warrant the installation 
of the necessary machinery for the production of pure caustic soda; 
consequently, without a market for this by-product, the cost of the 
chlorine would be the entire cost of the operation. In addition, a 
skilled chemical engineer who would be required would increase the 
cost per million gallons much more in a small plant than in a large 
one, while the uncertainty of the process and the increased respon¬ 
sibility on the sewage works both offset any slight advantage in cost 
which might appear in favor of the electrolytic plant. It has also 
been made clear in the present studies that the use of free chlorine, 
as contemplated in the earlier plan, is not economical and that some 
base should be provided for the preparation of hypochlorite. This 
base might be the caustic soda yielded by the process, or if a market 
for that, by-product were available, lime could be used. 

For the reasons outlined, therefore, this investigation of the possi¬ 
bilities of the electrolytic processes indicates that, contrary to earlier 


70 


DISINFECTION OF SEWAGE. 


views, such processes are not well adapted at the present time to 
sewage disinfection. Nor does it seem probable that hypochlorite 
from cells in which the products are allowed to recombine within the 
cell will ever be able to replace ordinary bleaching powder, 

CONCLUSION. 

The main reason for disinfecting sewage lies in the probable effect 
of discharging pathogenic bacteria into lakes, rivers, and harbors. 
Any such discharge is obnoxious to the sanitarian, and when the prac¬ 
tice of computing the cost of typhoid-fever epidemics becomes more 
general the cost of disinfecting sewage will not appear excessive. 
At present, however, the demand for sewage disinfection is confined 
to two conditions, namely, the possible pollution of water supplies 
and of shellfish beds. It has not yet been decided upon whom the 
responsibility rests for protecting domestic water supplies. The 
sanitarian recommends that rivers be kept as clean as possible and 
that water be filtered, but in practice distinction is made between 
supplies which are filtered before use and those which are not, and 
complete disinfection of sewage that enters streams from which sup¬ 
plies of the first kind are derived is still regarded as an unreasonable 
demand. The concensus of competent opinion requires at least, 
however, that, if an effluent is discharged within the region of im¬ 
portant shellfish beds or into a stream which is used as a source of 
domestic water supply without filtration, such effluent shall be free 
from pathogenic germs. Improved standards in sanitation and im¬ 
proved methods of disinfection will both operate to increase these 
minimum requirements, but in the meantime a thorough knowledge 
of disinfection methods and experimentation on the improvement and 
the cheapening of such methods will do much to hasten their general 
adoption. Slow sand filtration removes bacteria in a satisfactory 
manner and almost totally eliminates organic matter. Under certain 
conditions such a result is highly desirable, but the method is com¬ 
paratively costly, especially in the larger communities, and it is 
practically out of the question in many sections of the country. 

Chemical disinfection offers a means whereby a reasonable bacterial 
purification may be accomplished without complete purification of 
the organic matter. It is in no sense a substitute for sewage purifica¬ 
tion as ordinarily understood, for, though the application of chlorine 
compounds to an effluent oxidizes the organic matter in it to some 
extent and thereby increases its stability, such improvement is only 
incidental. It is not in anyway comparable with the cost of treat¬ 
ment, and it would be unwise to attempt to obtain stability, in such 
manner. Incidentally, however, the advantages of this increased 
stability are obtained, and it is probable that rapid sewage filters 


CONCLUSION. 


71 


may be worked at somewhat higher rates and with less margin for 
safe operation where chlorine treatment is employed as a finishing 
process. Under certain conditions it may be found desirable to 
effect bacterial removal without organic stability; along the seacoast 
and possibly along some great rivers of the Middle West the dilution 
factor is sufficienty high to preclude the danger of physical nuisance. 
Under other conditions the production of a stable effluent without 
regard to the amount of organic matter discharged may suffice; 
under others the removal of suspended matter may be of prime 
importance. Due consideration should be given in any particular 
process to the character of the organic matter in the effluent, and 
further treatment is advisable, not only where the discharge produces, 
or threatens to produce, an actual physical nuisance, but wherever 
the self-purifying powers of the stream will be appreciably drawn 
upon. In other words, stability is demanded in all effluents, unless 
the dilution is very great, not only in relation to the local discharge, 
but also in relation to all the sewage or effluent that the body of water 
in question may receive. This much is demanded from the stand¬ 
point of physical pollution alone. If, therefore, bacterial removal 
is also essential, disinfection is particularly satisfactory as a finishing 
process, because it can now be conducted at far less cost than the 
cheapest form of supplementary sand filtration. 

Comparison on a cost basis of the methods of chemical disinfection 
makes it apparent that chlorine in some form is the most efficient 
agent, though it must be admitted that the possibilities of heat and of 
certain organic compounds have not received adequate investigation. 
Calcium hypochlorite, or commercial bleaching powder, is by far the 
most satisfactory chlorine compound available. It has greater 
germicidal efficiency than equivalent amounts of free gaseous chlorine, 
chlorates, or perchlorates. It is equaled in efficiency by potassium 
and sodium hypochlorites, the products of certain electrolytic cells. 
The electrolytic production of hypochlorites or of free chlorine is not 
a satisfactory source of the disinfectant. The cost of such manufac¬ 
ture, on a scale necessarily small even at the larger sewage-disposal 
works, is at present so little below the cost of bleaching powder that 
no safe margin is left to cover the additional responsibility and uncer¬ 
tainty that are involved. Improved processes of manufacture may 
alter conditions somewhat, but the highest possible working efficiency 
and the cheapest power would not sufficiently reduce the costs to alter 
these general conclusions. 

The application of 3 parts per million of available chlorine in the 
form of bleaching powder to a trickling-filter effluent similar to those 
on which experiments were made effects satisfactory disinfection. 

* The removal of bacteria from the effluent averages over 95 per cent, 
making the removal for the whole purification process 98 to 99 per 


72 


DISINFECTION OF SEWAGE. 


cent of the number in the crude sewage. The cost of disinfection 
ranges from $1 to $1.50 per million gallons of sewage, depending 
chiefly on the size of the plant. Effluents of higher degrees of purity 
can be disinfected at still lower cost. Five parts per million probably 
represents the maximum amount of chlorine required for the treat¬ 
ment of trickling-filter effluents of poorer quality. The results 
obtained with the amounts of disinfectant that are specified do not, 
of course, amount to complete sterilization, but they may reasonably 
be called “practical disinfection.” Considerable additional cost is 
required to improve them but slightly. 

The disinfection of crude sewage to the same final condition 
requires the removal of over 98 per cent of its total bacteria. This 
may be accomplished by the application of from 5 to 10 parts per 
million of available chlorine, the amount depending on the character 
of the sewage. Such disinfection costs from $1.50 to $3.50 per 
million gallons. 

The disinfection of septic sewage requires the application of from 
10 to 15 parts per million of available chlorine. If no further puri¬ 
fication is required than that given by septic action and by disinfec¬ 
tion, it is advantageous to reverse the processes by disinfecting the 
crude sewage before it enters the tank. The resulting development 
of saprophytes within the tank has no sanitary significance, and it is 
doubtless of great advantage in the subsequent purification of the 
organic matter in the stream. 

The removal of B. coli is usually somewhat more complete than 
that of the total organisms. Under the conditions of a laboratory 
experiment, the results of hypochlorite disinfection on typhoid and 
colon bacilli in tap water were identical. It seems reasonable to 
assume, therefore, that the viability of the typhoid organism under 
working conditions in practical sewage disinfection is at least no 
greater than that of the colon bacillus and no greater than that of the 
sewage bacteria as a whole. Consequently the disinfection results 
obtained with total bacteria may, in the case of chlorine disinfection 
at least, be referred directly to the typhoid bacillus with assurance of 
reasonable accuracy. 

The results obtained in this investigation are so much more favor¬ 
able than any results that have been reported for similar work that 
comment on their accuracy and general applicability seems justifiable. 
The more important portions of the work have been practically 
duplicated under as diverse conditions as possible and by different 
workers. There is no apparent reason for believing that the results 
are not of general applicability. The reactions involved are partic¬ 
ularly free from interference on the part of the mineral constituents 
of normal sewage, a condition which has not been found where 
copper has been used as the disinfectant. The satisfactory results 


CONCLUSION. 


73 

in the present work are largely due to the fact that many little 
difficulties inherent in new processes have been overcome by con¬ 
tinuous work extending over considerable periods of time, and in 
particular to the fact that the experiments were made part of per¬ 
manent laboratory routine and were free from temporary and special 
characteristics which usually involve discontinuity and interruption. 
This routine continued week after week without interruption and 
without undue attention—in fact, just as it would naturally go on 
in practice. It is believed that the results represent what may be 
accomplished in practice and that they can be duplicated under 
working conditions on any scale which may be desirable. 


PUTRESCIBILITY AND STABILITY OF SEWAGE 
EFFLUENTS.” 


INTRODUCTION. 

The development of modern rapid processes of sewage treatment, 
involving the use of coarse material, has resulted in a somewhat 
changed conception of the functions of sewage disposal, while the 
general introduction of contact and trickling filters in the newer and 
larger works has made it necessary to examine methods of sewage 
analysis from a new viewpoint. Certain hitherto important features 
of the analysis have assumed comparatively unimportant roles, and 
new determinations have been developed on which the chief reliance 
is now placed. In the older methods of sewage purification, almost 
complete removal of organic matter and oxidation of the nitrogen 
were obtained, and the analytical methods employed in the control of 
such plants were designed to test their efficiency in accomplishing 
these ends. Consequently the determination of nitrogen in its 
different stages of oxidation and of organic matter in general were 
paramount. These determinations are only of minor importance in 
the practical control of modern rapid filters, where oxidation of 
nitrogen is incidental and removal of organic matter is but slight. 
Suspended solids, available oxygen, and the character of the effluent 
in reference to its stability now demand first consideration. As the 
production of a stable effluent is the primary function of such filters, 
the determination of stability becomes the most important analytical 
method in filter control. This point has been generally recognized, 
and incubation or putrescibility tests of one form or another are in 
general use, often at places where no further analyses are made. It 
unfortunately happens, however, that there are many different con¬ 
ceptions of what putrescibility really is and many different methods 
of determining it. Consequently statements of results lose much of 
their significance and comparisons are difficult or impossible. The 
present article is a review of the subject for the purpose of establish¬ 
ing the fundamental facts and of harmonizing current opinions; 
A method of determining stability, which has been, in use by the 
writer for nearly three years, and a numerical method of expressing 
results, by which quantitative value is given to the test, are also 
presented. 

a Investigation made at the sanitary research laboratory and sewage experiment station of the Massa¬ 
chusetts Institute of Technology. 

74 




PUTRESCIBILITY AND STABILITY OF EFFLUENTS. 


75 


PUTRESCIBILITY. 

Putrescibility, as applied to organic matter in general, implies the 
ability of that matter to undergo offensive putrefactive decomposi¬ 
tion. In a strict sense putrefaction is a term applied to nitrogenous 
matter only, though this is a popular rather than a logical conception. 
Exactly what constitutes offensive putrefactive decomposition in a 
sewage effluent is a matter on which opinions differ. Such decompo¬ 
sition is always anaerobic, and it is usually accompanied by the 
evolution of offensive odors. These two phenomena have, therefore, 
formed the basis of most putrescibility tests. Some criteria of 
putrefaction which have been employed are: (1) Development of 
offensive odors; (2) formation of black sediment; (3) reduction in 
the amount of dissolved oxygen; (4) loss of all dissolved oxygen; 
(5) loss of all available oxygen, including that of nitrates and nitrites; 
and (6) increase in the oxygen-consumed figure. Some of these 
tests are based on partial reduction of the available oxygen in the 
effluent; others depend on the complete reduction of the available 
oxygen and subsequent anaerobic fermentation. The tests most 
commonly employed belong to the latter group, depending on the 
production of odor or of hydrogen sulphide, blackening of the liquid, 
or reduction of organic dyes. The test which depends on an increase 
in the oxygen-consumed figure during incubation is also in that 
class, because anaerobic fermentation alone renders organic matter 
more readily oxidizable. 

These two types of test illustrate two distinct points of view 
which should be clearly differentiated. An effluent may be regarded 
as being composed of a given mass of organic matter dissolved or 
suspended in a definite amount of water. The water contains also 
a definite amount of available oxygen in the form of free dissolved 
oxygen, nitrites, nitrates, and possibly of other compounds. All the 
organic matter is oxidizable to some extent, and to that extent it 
serves as bacterial food. The greater the amount of organic mat¬ 
ter and the greater its oxidizability, the greater is the absorption 
of oxygen from the medium. Consequently a reduction of available 
oxygen in the effluent during incubation is a measure both of the 
amount of organic matter present and of its capability of oxidation. 
As a small amount of readily oxidizable matter has the same effect 
on the result as a larger amount of more stable matter, a test of 
this kind indicates whether or not the organic matter consumes 
oxygen; but it does not show whether or not the supply of available 
oxygen is sufficient to prevent the establishment of anaerobic condi¬ 
tions. This important question of the balance between the oxygen 
demanded by the organic matter and the oxygen available in the 


76 


PUTEESCIBILITY AND STABILITY OF EFFLUENTS. 


liquid is taken into consideration by tests of the second kind men¬ 
tioned, namely, those dependent on the establishment of anaerobic 
conditions. Such tests do not involve estimation of the amount 
and the kind of organic matter; indeed, organic matter which does 
not absorb any oxygen from the liquid under the conditions of an 
incubation'test must be very highly oxidized; and, furthermore, 
most organic matter derived from sewage is putrescible in itself — 
that is, if it is stored by itself in the absence of oxygen, it undergoes 
putrefactive changes. The question at issue is not, however, whether 
the organic matter itself will putrefy, but whether the effluent as a 
whole will become so reduced in oxygen that putrefaction will become 
possible. In other words, it is simply a question of a balance between 
the available oxygen of the effluent and the oxygen which the organic 
matter will require during the incubation period. It would seem 
that the problem might readily be solved by determining this bal¬ 
ance, but, unfortunately, it is not a simple matter, because the 
action involved is bacterial. Maiiy attempts have been made to 
determine the oxygen balance analytically, but such tests answer 
only with very good and very bad effluents, for which an inspection 
of the sample would serve just as well. When there is doubt about 
the character of the effluent — the condition for which such informa¬ 
tion is of most value — -all such analytical procedures have heretofore 
failed. It is evidently impossible to imitate with any degree of 
precision the bacterial activities that are involved. There remains, 
then, but one satisfactory expedient: To let the reaction proceed 
by itself and to note the result. But here also there are difficulties, 
because bacterial reactions of this sort are necessarily slow in reach¬ 
ing equilibrium, and the time required by a nicely balanced effluent 
is greater than can be allowed in routine work. Some arbitrary 
period of time, therefore, is usually adopted, and it is in respect 
to this factor that the confusion rises. If stability is to be con¬ 
sidered a definite qualitative characteristic of an effluent, that 
characteristic should be determined by a test sufficiently prolonged 
to insure equilibrium, but such procedure is not feasible for obvious 
practical reasons, and it is not desirable, because it is not enough 
simply to know that the available oxygen is sufficient or insufficient 
to satisfy the demands of the bacteria that are working on the 
organic matter. If the available oxygen is sufficient, there is perfect 
stability—a definite condition; if it is insufficient, there is still sta¬ 
bility in the quantitative sense—a relative stability determined by 
the relation of the available oxygen to the total amount of oxygen 
required by the organic matter for perfect stability. In practice 
the latter condition is the one usually encountered. 


PUTiySSCIBILITY AND STABILITY OF. EFFLUENTS. 77 

RELATIVE STABILITY. 

DEFINITIONS. 

The term putrescibility has had so many and so varied meanings 
in dictionaries, in popular parlance, and particularly in the minds 
of water chemists, that it is proposed to employ the word stability 
for that desirable quality which is the usual object of sewage purifica¬ 
tion—the transformation of the organic matter to such form that it 
is incapable of undergoing offensive putrefaction. This term has the 
added advantage of implying a positive characteristic that is acquired 
during purification, and it conveys a much more definite impression 
of the thing under discussion than the negative term 'putrescibility . 
A few more definitions are necessary in order to simplify the dis¬ 
cussion. The time required to establish anaerobic conditions in an 
effluent which, on incubation in a closed bottle, is subject to bacterial 
activities producing such conditions may be called for brevity the 
reducing time; the total amount of oxygen initially present in the 
form of free dissolved oxygen, nitrites, nitrates, and possibly other 
combinations may be called available oxygen; the term oxygen 
required for equilibrium or simply required oxygen may be understood 
to express the total amount of oxygen which would be consumed by 
bacterial action in the effluent if the latter were supplied with an 
unlimited amount of oxygen and if the reaction were allowed to 
proceed to a condition of substantial equilibrium. An effluent of the 
character under discussion is not stable in the absolute sense, because 
its available oxygen is less than the oxygen required for equilibrium; 
but, of two such effluents, that one is obviously the better which 
contains the greater amount of available oxygen in proportion to its 
required oxygen. In other words, effluents of this class have a certain 
relative stability which is indicated by the ratio of the available 
oxygen to the required oxygen. This relative stability, as will be 
shown, can be measured by the time required to reach the anaerobic 
stage. The term stability without qualification is employed in this 
paper to describe that condition in which the available oxygen exceeds 
the required oxygen, and the term relative stability is used to indi¬ 
cate the character of the effluent in the sense suggested. A perfectly 
stable effluent, therefore, has a relative stability of 100 per cent. 

It is apparent that time is an important element in stability tests, 
and that it is not compatible with the idea of relative stability to 
select an arbitrary period of time for establishing the line of de¬ 
marcation between stability and putrescibility. It is obviously 
unfair to record one effluent as nonstable because it “ holds up,” or 
fails to putrefy, for only three days and to record another as stable 


78 PUTRESCIBILITY AND STABILITY OF EFFLUENTS. 

4 

because it “holds up” for four days. A filter might deliver during 
one week an effluent that would fail to pass a four-day incubation 
test by a narrow margin and might deliver during the next week 
almost crude sewage for four days and a passable effluent for three. 
Obviously, the first week’s run would be the better and should be so 
recorded; but, under the present practice in many places, all the 
samples during the first week would be putrescible and 40 per cent 
of those during the second week would be nonputrescible. The first 
requisite, therefore, in logical study of the problem is that the time 
required for an effluent to reach a condition of anaerobic decomposi¬ 
tion shall be taken as an index of its relative stability. This time 
element is absolutely indispensable, and any test that is adopted for 
the determination of relative stability should be of such a character 
that the length of time required for the sample to reach a given 
anaerobic condition may be recorded. 

ESTIMATION OF THE REDUCING TIME. 

Many tests have been devised to determine whether or not an 
effluent is putrescible, and a review of the subject with the details 
of the methods proposed has been given elsewhere." In any deter¬ 
mination of the time required to exhaust the available oxygen in an 
effluent the following conditions must be fulfilled: (a) The sample 
must completely fill the bottle, the stopper of which must be tight 
and must not be removed during the test; (b) determinations must 
be made at a standard temperature; (c) observations must be made 
at least as frequently as once a day. If a test is employed which 
necessitates opening the bottle in order to observe the condition of 
the sample; one bottle must be incubated for each day that the effluent 
is under observation. This is true for any test which depends on a 
chemical determination of any constituent or which depends merely on 
the odor developed. Obviously such tests are not well adapted to the 
conditions heretofore stated. A simpler procedure is one in which 
the anaerobic condition can be detected by the appearance of the 
effluent without opening the bottle. The anaerobic fermentation that 
occurs immediately after the complete exhaustion of the oxygen 
is usually accompanied by a production of hydrogen sulphide. 
Consequently an indicator that is sensitive to hydrogen sulphide 
is advantageous in detecting the beginning of the anaerobic fer¬ 
mentation. Fortunately delicate indicators are available for this 
purpose, for certain organic dyes are readily reduced to correspond¬ 
ing leuco-compounds under anaerobic conditions. Methylene blue 
is an organic dye of this character, and it is reduced to its colorless 
leuco base by hydrogen sulphide, alkaline sulphides, and by the 

a Phelps, E. B., and Winslow, C.-E. A., On the use of methylene blue in testing sewage effluents: Jour. 
Infectious Diseases, Suppl. No. 3, 1907, p. 1. 




KELATIVE STABILITY. 


79 


commercial reducers used in dyeing. Its employment for the study 
of stream pollution was first proposed by Spitta,® and it was later 
more thoroughly investigated by Spitta and Weldert b as a test for 
sewage effluents. 

Methylene blue, or tetra-methyl-thionin chloride, is a commercial 
dye of complex constitution, having the empirical composition 
Ci 6 H 18 N 5 SC1. Merck’s medicinal preparation is pure and it is prefer¬ 
able to the commercial article for sewage work. It is an extremely 
sensitive indicator for hydrogen sulphide and other reducing bodies, 
being decolorized at once in the presence of even small traces; its 
decolorization by bacterial action has been studied by many observers, 
the principal of whom are cited by Spitta and Weldert. * 6 The 
technique of its use in testing a sewage effluent is extremely simple. 
One cubic centimeter of a one-tenth per cent aqueous solution of the 
dye is added to the effluent in a glass-stoppered bottle of 250 cubic 
centimeters capacity, and the sample is then incubated either at 20 ° C. 
or at 37° C. The blue color of the solution remains practically 
unchanged till the available oxygen contained in it has been consumed 
and putrefactive conditions have been established. At this stage 
the dye is reduced and decolorized. The time required for such 
decolorization is, therefore, approximately the time required for the 
exhaustion of the available oxygen. The dye is an indicator for what 
may be called the oxygen neutral point, the point at which the avail¬ 
able oxygen becomes exhausted and anaerobic conditions are estab¬ 
lished. Some studies d made at the sewage experiment station 
confirm the earlier conclusion of Spitta and Weldert, that the end 
point indicated by this dye is almost exactly the desired neutral 
point. The order in which different forms of oxygen are reduced 
appears to be: Dissolved oxygen, nitrates, nitrites, sulphates, and 
phosphates. Methylene blue was found to change color practically 
at the same time as the nitrites in this series. It is readily conceivable 
that an indicator might possess such properties that it would be 
reduced before the nitrates or even before the total exhaustion of the 
free oxygen. Another indicator might change only after the reduc¬ 
tion of the sulphates. The fact of this varying end point has been 
well shown in a recent paper by Clark and Adams. 6 Comparative 
incubation tests were made with 17 dyes as indicators, only 6 of which 
were reduced during the incubation. Arranged in the order of their 
reducibility these are: Indigo carmine (sulphonated indigo), methyl- 

a Archiv. fur Hyg., 1903, vol. 46, p. 113. 

6 Spitta and Weldert, Mitteilungen aus der Koniglichen Prufungsanstalt fur Wasserversorgung und 
Abwasserbeseitigung zu Berlin, vol. 6, 1906, p. 161. 

« Loc. cit. 

d Phelps, E. B., and Winslow, C-E. A., On the use of methylene blue in testing sewage effluents: Jour. 
Infectious Diseases, Suppl. No. 3,1907, pp. 1-13. 

« Clark, H.W., and Adams, G. O., Studies in incubation tests: Jour. Am. Chem. Soc., vol. 30,1908, p. 1037. 

76474—irr 229—09-6 



80 


PUTRESCIBILITY AND STABILITY OF EFFLUENTS. 


ene green, and methylene blue; and then, order not stated, congo 
red,- methyl orange, and tropseolin. The average time computed 
from 26 tests for the reduction of indigo carmine was 2 days; for 
methylene green the average time was 2.4 days; and for methylene 
blue, 3.9 days. As it is of course impossible to hasten or to retard 
the reactions that are taking place, these differences show differences 
in the end point recorded by the several indicators. 

In the writer’s experiments it was shown that the end point 
indicated by methylene blue is probably that point at which the 
free oxygen and the nitrates are practically exhausted and reduction 
of the sulphates is just beginning. This is understood to be the point 
at which anaerobic conditions are established. The work was not 
undertaken, however, for the purpose of determining the end point 
accurately, and it is possible that the end point of methylene blue 
is a little too far along and that either methylene green or indigotin 
indicates the desired point more closely. Just as it is essential in 
other branches of analysis to specify the indicator that shall be used 
in a given determination, in order to prevent confusion in compara¬ 
tive work, similarly it is important in this test to adopt a standard 
indicator as a basis of comparison. The results of all experiments 
thus far are in favor of methylene blue, and that dye is now widely 
and satisfactorily used in the laboratories of the country; conse¬ 
quently, its retention as a standard appears advisable, at least until 
further experimental evidence is available. The present series of 
comparative stability values is calculated on the assumption that 
methylene blue be used. A change in the final end point adopted 
would, of course, necessitate a remodeling of the computations. 

THEORETICAL RELATION BETWEEN REDUCING TIME AND 
RELATIVE STABILITY. 

The time required for complete exhaustion of the oxygen from 
an effluent — the reducing time — is obviously not a simple function 
of its relative stability; but there is a well-known theoretical relation 
between velocity of reaction and amount of reacting substance, 
from which it is possible to compute one from the other. It is a 
principle of physical chemistry that the velocity of a chemical reaction 
is a function of some power of the concentrations of the reacting 
substances. In the simplest cases the velocity varies directly as 
the concentration. The bacterial reactions that have been investi¬ 
gated also conform to this law,® and it has been, therefore, applied to 

a Chick, H., An investigation of the laws of disinfection: Jour. Hyg., vol. 8,1908, p. 92. 

Lubenau, C., Zur Saiirebildung der Diphtheriebazillen: Arch. f. Hyg., vol. 66,1908, p. 305. 

Famalener, W., and Madsen, T., Die Abschwiichung der Antigen dureh Erwarmung: Bichem. Zeitung, 
vol 11, 1908, p. 186. 

Nawiasky, P., Uber die Umsetzung von Aminosaiiren durch B.proteus vulgaris: Arch. f. Hyg., vol. 66, 
1908,p. 209. 



RELATIVE STABILITY. 


81 


the present study. The exact expression of the function need not 
be detailed, for it can easily be seen that this general law is approxi¬ 
mately applicable; if one-half the work of oxidation is accomplished 
in one day, the availability of the organic matter as a food supply 
is reduced one-half, and the consequent bacterial activity on the 
organic matter is reduced accordingly; at the beginning of the 
second day food supply and bacterial activity are in the same rela¬ 
tive proportions and the same relative amount of work is done again; 
that is, one-half of the remaining work of oxidation will be done 
during the second day. This law is expressed by the following 
equation: 

log. x = log. a — T& 

in which a is the amount of oxygen required for equilibrium at the 
commencement of the action, and x is the amount similarly required 
at the end of the time t, while Jc is a constant known as the velocity 
constant. If a grams of oxygen are required for initial equilibrium, 
and if x grams are required after the sample has been incubated in 
a tight bottle for a period of time which may be termed t days, 
(a — x) grams of oxygen have been abstracted from the liquid by the 
organic matter. If the available oxygen of the liquid has just 
become completely exhausted at the end of t days, the value (a — x) 
represents the amount of available oxygen originally present in the 
sample. It is not even necessary to know the actual amounts of 
oxygen, because the ratio of available oxygen to the oxygen initially 
required for equilibrium gives a relative stability factor that obviates 
the expression of the actual amounts. The ratio is 

a — x 
a 

The value of this expression in terms of t and h can be found by 
using the logarithmic equation given above: 



The second term of this equation, therefore, is equal to the ratio 
between the total available oxygen and the oxygen required for 
equilibrium, the ratio being expressed in terms of a constant and the 
reducing time. This ratio is the relative stability. 

If a and x could be determined by analysis it would be possible to 
determine the value of Tc by a few tests. It has already been shown, 
however, that there is no simple chemical method of determining 
the amount of oxygen which is consumed by the organic matter 
under natural conditions. A possible method involves actual 
measurement of the amount of oxygen absorbed by the liquid in a 
time sufficiently prolonged to insure virtual equilibrium. If, how- 


82 


PUTRESCIBILITY AND STABILITY OF EFFLUENTS. 


ever, ic is determined only approximately and if a series of values for 
(1— W) is obtained for all values of t, the terms of this series bear 
practically a constant relation to each other, even if & is varied con¬ 
siderably. The number expressing the relative stability is, in any 
case, a true index of the character of the effluent, independently of the 
further requirement that it shall be the absolute ratio of the available 
oxygen to the required oxygen. If it is not this absolute ratio it 
stands in constant but unknown relation to it. 

An indirect method has been devised for determining the value 
of Tc with a degree of accuracy which is ample for the present dis¬ 
cussion. For this purpose the results of 2,649 separate stability 
tests have been analyzed. The nature of these samples and the 
manner of determining the reducing time t is described on pages 78 
to 80. It is sufficient to state that the reducing time was from one 
to twenty days in most of the tests, while many of the samples showed 
a relative stability of more than 100 per cent. As the samples which 
required a reducing time between one and twenty days had relative 
stabilities ranging from zero to 100 per cent, it was assumed for the 
purpose of approximating the value of Tc that one-half had values 
less than 50 per cent and one-half values greater than 50 per cent. 
Such assumption, of course, is justifiable only with a large number of 
observations, but it is believed that it is sufficiently accurate in the 
present case. Inspection of the tabulated results showed that a 
stability of 50 per cent was attained, at a temperature of 20° C., in 
almost exactly three days, thus making 

l-fc 3 = 0.50; or & = 0.794. 

If this value of Tc is substituted in the equation before mentioned, 
the following values of the relative stability corresponding to the time 
t in days are obtained. 

Table 1. — Relation between reducing time and relative stability at 20° C. 


Reducing 
time in days. 
(*■) 

Relative 
stability. 
(1—0.794*.) 

Reducing 
time in days. 

' (<.) 

Relative 

stability. 

(1—0.794*.) 

1 

21 

9 

87 

2 

37 

- 10 

90 

3 

50 

11 

92 

4 

60 

12 

94 

5 

68 

14 

96 

6 

75 

16 

97 

7 

80 

18 

98 

8 

84 

20 

99 


These values of relative stability are strict measures of the char¬ 
acter of the effluent. An effluent which contains more than sufficient 
oxygen to establish stability should have a stability of 100; in other 
words, it would never reach the anaerobic stage. In practice it is 
necessary to set some time limit to the tests and give an average 








RELATIVE STABILITY. 


83 


stability value to all tests exceeding this limit. This value is suf¬ 
ficiently high to indicate the character of the effluent. On the 
other hand, crude sewage containing a little dissolved oxygen is 
completely reduced in one or two hours, or, if it contains no dissolved 
oxygen, decolorizes methylene blue at once; in numerical expres¬ 
sion its relative stability is practically zero. These figures are com¬ 
parative, because they may be added and divided to obtain periodical 
averages and the averages thus obtained are properly weighted. 
This is not so if the reducing times themselves are averaged. The 
figures are also an approximate measure of the ratio between the 
total available oxygen in the effluent and the amount of oxygen 
required for the production of stable equilibrium in the organic 
matter, and it is believed that the approximation is sufficiently close 
for ordinary purposes of interpretation. 

DETERMINATION OF RELATIVE STABILITY. 

INCUBATION PERIODS. 

Relative stability as previously defined is a numerical measure of 
the relation between available oxygen and required oxygen and it is 
also a function of the reducing time. Some practical applications of 
the stability values in Table 1 remain to be outlined. 

Incubation periods exceeding five days are inconvenient and 
probably unnecessary in practical work, but in experimental work 
where more detailed knowledge is desired, longer periods may be 
used tn advantage. At the sewage experiment station, a fourteen- 
day period has been adopted. During the past two years tests have 
been made of more than 2,600 samples of trickling-filter effluents of 
such a quality that most of them were near the border line between 
satisfactory and unsatisfactory effluents. Most of them were on the 
safe side, but some of them were unsatisfactory for considerable 
periods. The results of this large number of tests, therefore, consti¬ 
tute an admirable basis for studying the advisability of using shorter 
periods of incubation. In the summary of the results in Table 2 
the tests are divided first in ten groups corresponding to the different 
filters and the years during which the tests were made. Effluents 
A, B, C, and D are from trickling filters before sedimentation; E and 
F are the same effluents after two-hour sedimentation. The results 
in each group are subdivided in order to show the per cent of the 
number of samples that retained available oxygen at the end .of 
stated periods. The results shown in this table have been platted 
and from' the plats the number of samples which would have retained 
available oxygen after twenty days have been determined by exter- 
polation from a plat of the results for shorter periods of time. 


84 


PUTRESCIBILITY ANI> STABILITY OF EFFLUENTS. 


Table 2. Summary of stability tests of trickling-filter effluents, showing time required 
to exhaust the available oxygen.a 


Num¬ 
ber of 
sam¬ 
ples 
tested. 


Per cent of samples retaining oxygen after— 


1 

day. 


2 

days. 


3 

days. 


4 

days. 


6 

days. 


days. 


10 

days. 


12 

days. 


14 

days. 


206 

days. 


35 

45 

69 

71 

14 

3 

1 

0 

0 

1 


a Indicator, methylene blue; temperature of incubation, 20° C. 


6 Values exterpolated. 


If it is assumed that in practice an incubation period of four days 
at 20° C. will be employed, it is possible to show from the data in 
Table 2 what relative stability values should be assigned to samples 
that retain some available oxygen after four days. Such samples 
may be divided into two hypothetical groups, namely, those which 
would have lost their oxygen between four and twenty days and 
those which would have retained it more than twenty days. Obvi¬ 
ously a relative stability of 100 may be assigned to the latter because 
the value at twenty days is 99. An average time of ten days may 
be selected for the period between four and twenty days, thus mak¬ 
ing proper allowance for the decreasing number of samples, which 
reduce each day, and a relative stability of 90 may be assigned to that 
group. Between 43 and 63 per cent of the samples which passed the 
four-day period were reduced before twenty days. This per cent 
varied with the quality of the effluent, but it may be taken as 50 per 
cent without introducing an error of more than 5 per cent in the final 
result in any group. For practical purposes it may be assumed that 
one-half of all tests passing a four-day period of incubation will be 
reduced before twenty days, or in an average time of ten days, and 
that one-half will exceed twenty days. This gives 95 for an aver¬ 
age relative stability value of the whole number of samples. If a 
four-day period of incubation at 20° C. is adopted, each sample 
reduced on the first, second, third, or fourth day may be recorded 
as having a relative stability of 20, 37, 50, and 60, respectively, and 
all samples retaining available oxygen after four days may be given 
a relative stability of 95. An individual result obtained in this 
manner will have but little accuracy, and when only a few tests are 
made, an incubation period of at least 10 days should be employed. 
On the other hand, when daily tests are made, the method outlined 
will give for monthly periods average results that are very close to 
the truth. Relative stability values calculated from the data in 



































RELATIVE STABILITY. 


85 


Table 2 for the entire twenty-day period of incubation are com¬ 
pared in Table 3 with similar values obtained by the shorter, four-day, 
method. The agreement is satisfactory. If smaller numbers of 
tests are considered, the distribution naturally will not be so regular 
and greater errors will be introduced in the results calculated. In 
order to determine the accuracy of the proposed method, the first 
776 samples of Table 2 have been subdivided into twelve quarterly 
groups, containing from 36 to 78 samples in each group, and the 
relative stability values have been calculated for these smaller 
groups, as shown in Table 4. 

Table 3. —Comparison of relative stability results obtained from the twenty-day incubation 
period shown in Table 2 with those calculated from a four-day period. 


A 

B 

E 

F. 

A 

B 

C. 

D 

E 

F. 


Effluent. 


Period. 


>1906-7 


1907-8 


Relative stability. 

20-day 

4-day 

period. 

period. 

f 76 

77 

81 

82 

88 

86 

86 

84 

43 

43 

39 

41 

31 

31 

25 

25 

38 

39 

28 

28 


Table 4.— Comparison of relative stability results obtained from smaller numbers of 
samples by the use of four, six, and twenty day incubation periods. 


Effluent. 

Quarter. 

Number 

of 

samples 

averaged. 

Relative 

4-day . 
period. 

stability bas 

• 6-day 
period. 

ed on a — 

20-day 

period. 


1906. 







[First. 

37 

59 

59 

58 



Second. 

73 

57 

53 

52 

* 


Third. 

59 

64 

61 

60 

A. 


Fourth. 

77 

89 

88 

88 



1907. 







First. 

74 

92 

92 

92 



Second...:.. 

68 

91 

91 

92 


1906. 







[First. 

36 

62 

61 

59 



Second. 

71 

75 

69 

69 



Third. 

59 

81 

79 

82 

B. 


Fourth.... 

78 

86 

84 

90 



1907. 



First. 

75 

91 

90 

95 



Second... 

69 

91 

90 

93 


The maximum error with the smaller groups is about 10 per cent, 
and this error can be materially reduced by basing the calculation 
on a somewhat longer incubation period. The second column in 
Table 4 shows the relative stability values calculated from a six-day 























































86 PUTRESCIBILITY AND STABILITY OF EFFLUENTS. 

period, and the maximum error in this computation is less than 5 per 
cent. If the greater number of the samples of a well-purified effluent 
retain oxygen after four days, the results with any short method of 
calculation will be low. Since a value of over 95 can not be obtained 
with this method, it is obviously impossible to provide a four or six 
day method which will give accurate results. If more accurate infor¬ 
mation is necessary, longer periods of incubation—perhaps as long as 
ten days—are advisable. It matters little, however, whether the rela¬ 
tive stability is 95 or 99, as far as practical results are concerned, 
because either value represents a high degree of purification. 

EFFECT OF TEMPERATURE ON THE REDUCING TIME. 

It has been assumed in the foregoing discussion that the tempera¬ 
ture of incubation is 20° C., and there are certain reasons why this 
temperature is better than higher ones. It probably represents 
more nearly than any other th^ average temperature of streams, 
and results depending on bacterial activity should be obtained near 
the normal temperature to which the bacteria are exposed in nature. 
Incubations at 37° C., for example, probably cause the development 
of a class of bacteria quite different from those normally at work. 
The most serious objection to higher temperatures, however, is the 
fact that certain effluents, particularly those from the rapid filters 
which are in such common use, contain a large amount of dissolved 
oxygen, frequently 8 or 10 parts per million, and this represents the 
greater part of the available oxygen. At 20° C. 9 parts per million 
of oxygen is the saturation point. The saturation point at 37° C. 
has not been determined, but it is not over 7 parts per million, so 
that there is a tendency for some of the dissolved oxygen to escape 
at high temperatures. Tight stoppers will not prevent the escape 
of this released gas unless great care is taken with them, and 
mercury seals are hardly adapted to routine work. It is therefore 
practically impossible properly to maintain the necessary saturation 
conditions at a temperature higher than 20° C. A variation of 
temperature with season would have certain advantages, but such 
adjustment is rather impracticable. Since results are frequently 
obtained at 37° C., the relation of such results to those obtained at 
20° C. has been investigated and has already been reported.® Vari¬ 
able relations were found, as was expected, but the average results 
may be used with some degree of accuracy. It was found that the 
time of reduction at 37° C. was from 37 per cent to 72 per cent of 
that required at 20° C., and the mean of 20 determinations was 
exactly 50 per cent. Carefully fitted glass stoppers were used in 
the bottles, and ordinary precaution was taken to prevent loss of 


a Phelps, E. B., and Winslow, C. E.-A., loc. cit. 




RELATIVE STABILITY. 


87 


oxygen. If haste is necessary, a temperature of 37° C. will give 
results in about one-half the time required at 20° C. But the use 
of a temperature of 20° is strongly recommended whenever it is 
possible to employ it. 

SUMMARY OF METHOD. -v 

Samples should be collected in glass-stoppered bottles of 150 or 
200 cubic centimeters capacity. No special precautions are neces¬ 
sary in collecting samples of ordinarily good effluents that are fairly 
high in dissolved oxygen. If the dissolved oxygen is low, precau¬ 
tions similar to those used in collecting dissolved oxygen samples 
should be observed. A one-tenth per cent solution of methylene 
blue, preferably Merck’s medicinal quality, is used as indicator. One 
cubic centimeter of this solution is added to each of the samples, 
which are then incubated, preferably at 20° C., for four days, and 
observations are made at least once a day. The samples in which the 
methylene blue becomes decolorized are recorded as having a rela¬ 
tive stability corresponding to the time required for reduction. 
Those that are blue at the end of four days are given a relative 
stability value of 95. 

Table 5 gives the relation between time for reduction and the rela¬ 
tive stability. Though the figures up to four days are the only 
ones required, the entire series up to twenty days is given for 
comparison. Relative figures for incubation at 37° C. also are given, 
but the use of that column is not recommended, except when it is 
absolutely necessary to use the higher temperature, or when it is 
desired to convert results to a standard basis of 20° C. 

Table 5.— Relation between relative stability and reducing time at 20° and at 37° C. 


Relative stability numbers.** 

Relative stabilty numbers.** 

ho. 

*37. 

s. 

h o. 

<37. 

s . 

0.5 


11 

8.0 

4.0 

84 

1.0 

0.5 

21 

9.0 

4.5 

87 

1.5 


30 

10.0 

5.0 

90 

2.0 

1.0 

37 

11.0 

5.5 

92 

2.5 


44 

12.0 

6.0 

94 

3.0 

1.5 

50 

13.0 

6.5 

95 

4.0 

2.0 

60 

14.0 

7.0 

96 

5.0 

2.5 

68 

16.0 

8.0 

97 

6.0 

3.0 

75 

18.0 

9.0 

98 

7.0 

3.5 

80 

20 

10 

99 


a-S=Relative stability or ratio of available oxygen to oxygen required for equilibrium? Expressed in 
>er cent. 

< 20 = Time in days to decolorize methylene blue at 20° C. 

<37= Time to decolorize at 37° C. 

Theoretical relation — 
s=100 (1-0.794 n o) 

=100 (1-0.630 W) 




















88 


PUTRESCIBILITY AND STABILITY OF EFFLUENTS. 


INTERPRETATION OF RESULTS. 

The arbitrary tests for putrescibility now employed require no 
interpretation. An effluent is either putrescible or nonputrescible, 
according to whether it is on one side or the other of a certain line 
of demarcation. The fact that the dividing lines, for there are 
many of them, are perfectly arbitrary and have no real significance 
seems to have been overlooked. Fixed arbitrary standards in sewage 
analysis are undesirable in that they relieve the analyst of the impor¬ 
tant and difficult duty of interpreting his own results in the light of 
his own peculiar environment. Some general observations and rules 
may be stated for aid in interpretation, but it should always be 
borne in mind that such interpretations are dependent as much on out¬ 
side conditions as on the analyses themselves. The mere statement, 
therefore, that 75 per cent of the samples of a tested effluent were non¬ 
putrescible has no bearing whatever on the broader phases of inter¬ 
pretation. These facts are well known and accepted in ordinary 
analytical features. The aim of the present investigation has been 
to place the important determination of stability on the same basis. 
Under this proposed method of determining relative stability an 
interpretation of the information in hand is rendered possible. A 
relative stability of 75 per cent means that the effluent in question 
contains a supply of available oxygen equal to 75 per cent of the 
amount of oxygen which the effluent will eventually require before 
it will have become perfectly stable. The amount of this available 
oxygen is estimated fairly well by the chemical determination of 
dissolved oxygen and nitrates. The nitrites are usually so low that 
they are negligible, and it is unnecessary to decide whether or not 
the nitrates represent available oxygen, because they have been 
included in the test and must be considered in the interpretation. 
Undoubtedly the nitrates will not be used in the stream until the 
dissolved oxygen of the water has been reduced to a low point. 
Nevertheless, the fact remains that the available oxygen in the 
effluent, including the nitrates, is 75 per cent of that required for 
equilibrium, and that the remainder must come from the water of 
the stream, which must also supply enough additional oxygen to 
replace that which may be abstracted from the nitrates of the effluent, 
if aerobic conditions are to be maintained. Analyses of water from 
the stream and estimates of the relative volumes of the stream and 
the effluent complete the data necessary for a full interpretation. 

In general, effluents having a relative stability greater than 90 per 
cent may be discharged into any stream without danger of their 
consuming any of the oxygen of the water, because effluents of such 
high stability will retain oxygen indefinitely on exposure to the air. 


INDEX 


Page. 

Acids, use of, for disinfection.16-17 

Alkalinity, effect of, on copper sulphate 

process. 31 

Amines process, description of.32-33 

B. 

Bacteria, fate of, in purification. 8-12 

purification by. * . 7 

Bacillus coli, persistency of. 10, 

12,22-23,36,62-63,72 

tests for, records of. 37 

Bacillus pyocyaneus, persistency of. 12 

Bacillus sporagenes, persistency of. 10 

Bacillus typhus, persistency of.9,62-63 

Bacteria, sedimentation of.45^6 

Bacteriological methods, description of.35-36 

Baltimore, Md., experiments at.12,14,56-60 

Baltimore sewerage commission, cooperation 

of.35,56 

Bengal, India, experiments at.24-25 

Berlin, experiments at.19,21-24 

Birge, E. G., work of. 35 

Bleaching powder. See Chloride of lime. 

Boston, investigations at.37-51 

investigations at, character of.37-38 

Brewster, N. Y., experiments at. 27 

C. 

Calmette, A., experiments of. 11 

Carpenter, W. T., work of. 35 

Chemical disinfection, cost of. 14 

nature of. 70-71 

Chesapeake Bay, pollution of. 13-14 

Chloride of lime, amount needed of. 38-39, 

40,43,46,48-51,59,63-65 
amount needed of, relation of tempera¬ 
ture to. 43 

contact period needed for. 38-39, 

45-46,48-51,64-65 

disinfection by, cost of. 65-70,71-72 

effect of, on colon and typhoid bacilli_62-63 

evenness of, results with. 43-45,58-59 

experiments with.38-51 

penetration of solid particles by .22-23 

price of.65-66 

use of, as disinfectant. 18-24,71 

Chlorine, electrolytic manufacture of.25-29 

exhaustion of. 62 

germicidal action of.17,60-62 

use of, as disinfectant. 17-29,34,71 

Chlorine, available, definition of. 18 

Chlorine cells, description of. 69 

Chlorine compounds, germicidal action of_60-62 

Chlorine gas, germicidal action of.60-61 

production of. 17-18 I 


Page. 

Chloros, nature of.19,67 

Clark, H. W., and Adams, G. O., on reducing 

time. 79 

Columbus, Ohio, experiments at. 11,29-31 

Contact-filter effluents, bacteria in. 11 

effect of chloride of lime on. 25 

effect of copper sulphate on.30,32 

Contact period, length of, in chloride of lime 

process. 38-39,44-45 

length of, in copper sulphate process. 30 

Copeland, W. It., and Johnson, G. A., exper¬ 
iments by.29-31 

Copper, use of, for disinfection. 29-32,34 

Copper sulphate, disinfection by, relation of 

temperature to.30,43 

Cowles, R. P., work of. 35 

Crawford, H. S., work of. 34 

Crude sewage. See Sewage. 

D. 

Daniels, F. E., work of.7,34,51 

Delaware River, pollution of. 14 

Dibdin, W. J., experiments of. 19 

Digby, W. P., experiments of. 67 

Disease, effect of filter purification on. 10 

Disinfection, degree of. 24 

applications of.63-65 

conclusions on..70-73 

costs of. 65-70 

methods of. 15-34 

classification of. 15 

See also particular methods. 

necessi ty for.;. 12-15 

See also Purification: Stability. 

Drinking water, purification of, responsibility 

for.. 11-12,70 

Drown, T. M., on electrolytic process. 26 

Dunbar and Korn, experiments on. 19 

Dunbar and Zirn, experiments of. 19 

E. 

Eau de Javelle, nature of. 19 

Electrolytic chlorine processes, cost of. 66,69-70,71 

description of. 25-29,67-69 

Eisner and Proskauer, experiments of. 19 

Emerson, C. A., work of. 35 

Experimental investigations, account of.34-65 

expression of. 36-37 

F. 

Fermi, experiments of. 27 

Filter effluents, effect of chloride of lime on.... 25 

effect of copper sulphate on.30-32 

effect of sodium benzoate on. 33 

Filtration, effect of, on bacteria. 9 

Fuller, G. W., on oyster crop. 13 


89 























































































90 


INDEX 


Page. 

Gelatine, penetration of, by chloride of lime.. 22-23 


Germany, chemical disinfection in. 14 

H. 

Hamburg, experiments at. 19-21 

Heat, use of, for disinfection. 15-16,33 

Ilermite process, description of.27-28 

Hospital sewage, experiments on. 20 

Houston, A. C., experiments of.9-10,12 

Hypochlorites, use of, in disinfection.18-25 

I. 

Ivanoff, investigations of. 16 

J. 

t 

Jackson, D. D., bile broth test of. 11,35 

Johnson, G. A., experiments of. 11 

Johnson, G. A., and Copeland, W. R., experi¬ 
ments by. 29-31 


K. 

Kellerman, K. F., and Moore, G. T., experi¬ 
ments of. 29 

Kellerman, K. F., Pratt, R. W., and Kim¬ 
berly, A. E., experiments by. 25,31-32 


Kitasato, investigations of. 16 

Klein, E. E., experiments of.15-16,33 

Korn and Dunbar, experiments of. 19 

Kranepuhl and Kurpjuweit, experiments of. 21-22 

L. 

Labarraque’s solution, nature of. 19 

Lillis, M. H., work of. 35 

Lime, use of, for disinfection. 16 

M. 

MacConkey, Alfred, experiments of. 9 

McDonald Electrolytic Cell Co., aid of-. 35 

McLintock, experiments of. 27 

McRae, H. C., work of.-.. 35 

Marion, Ohio, experiments at. 25 

Massachusetts Institute of Technology, coop¬ 
eration of. 34 

Moore, G. T., and Kellerman, K. F., experi¬ 
ments of. 29 

N. 

New Jersey State Sewerage Commission, co¬ 
operation of. 34 

O. 

Ohio, experiments in. 25 

Organic matter, effect of, oncopper sulphate 

process. 31 

Oxychloride process, description of.28-29 

Oxygen, available, definition of. 77 

Oxygen, required, definition of. 77 

Ozone, use of, as disinfectant. 17 

P. 

Pathogens, fate of, in purification. 8-10 

Permanganate of potash, use of, for disinfec¬ 
tion. 32 

Phelps, E. B., work of. 7 

Pickard, experiments of. 9 

Plainfield, N. J., experiments at.11-12 


Page. 

Potassium chlorate, germicidal action of.60-61 

Potassium hypochlorite, germicidal action of. 60-61 
Potassium perchlorate, germicidal action of.. 60-61 
Pratt, R. W., Kimberly, A. E., and Keller¬ 
man, K. F., experiments by.. 25,31-32 
Pritzkow, A., and Thumm, K., experiments 


of. 11 

Proskauer and Eisner, experiments of. 19 

Purification, agents of. 7-8 

essentials of. 12,74 

significance of term. 8 

speed of. 7-8 

improvement in.... 7-8 

See also Stability; Disinfection. 

Putrefaction, prevention of. 8 

Putrescibility, arbitrary standards of. 88 

control of..... 74 

tests of. 75-76,88 

R. 

Red Bank, N. J., experiments at.51-56 

sewage-disposal works at. 52 

plate showing. 52 

Reducing time, definition of. 77 

estimation of... 78-80,83-86 

stability and, relation of.80-83 

temperature and, relation of.86-87 

Relative stability. See Stability, relative. 

Rideal, Samuel, investigations of... 9,16,28 

S. 

Sand-filter effluent, bacteria in. 11 

effect of chloride of lime on. 25 

effect of copper sulphate on.30-32 

Sand filtration, effect of. 70 

Schumacher, experiments of. 19-20 

Schwarz, investigations of. 20-21 

Septic effluent, chemical composition of. 54 

disinfection of, cost of.55-56 

effect of chloride of lime on.. 24-25,51-56,64,72 

effect of contact filter on. 11 

effect of oxychloride on.j.28-29 

effect of sand filtration on. 11 

effect of trickling filter on. 11 

treatment of, by chloride of lime.24-25 

Septic tank, effect of. 9 

Sewage, crude, disinfection of. 46-47 

disinfection of, cost of. 55-56 

effect of chloride of lime on. 19- 

22,23,26,46-51,64, 72 

effect of contact filter on. 11 

effect of copper sulphate on.30-32 

effect of oxychloride on.28-29 

effect of sand filter on. 11 

effect of trickling filter on. 11 

experiments on, in Germany.23-24 

sedimentation of. 47 

superabundance of, effect of, on streams. 8 

treatment of, by chloride of lime.23-24 

Shellfish beds, protection of. 13-14,70 

Sodium benzoate, use of, for disinfection..... 33 

Sodium hydroxide, germicidal action of.60-61 

Sodium hypochlorite, germicidal action of... 60-61 
Sodium permanganate, use of, in disinfection. 19,32 

Soil bacteria, purification by.. 7 

Sprinkling filter effluent, effect of copper sul¬ 
phate on. 30 




























































































INDEX. 


91 


Page. 

Stability, definition of.77,88 

necessity for. 74 

relation of time and. 77-78,80-83 

Stability, relative, definition of. 77,85-86 

determination of.83-86 

Streams, purification of. 8,13 

Stutzer, investigations of. 16 

T. 

Temperature, effect of, on disinfection. 43 

effect of, on reducing time.86-87 

Thumm, K., and Pritzkow, A., experiments 

of. 11 

Tonnetta Creek, N. Y., experiments on. 27 

Trickling-filter effluents, bacteria in. 11 

chemical composition of. 40 

effect of chloride of lime on. 38- 


46,56-60,63-65,71-72 


Page. 

Trickling-filter effluents, effect of copper sul¬ 


phate on.30-31 

effect of sodium benzoate on. 33 

V. 

Vibrio, persistency of.'_ 21 

W. 

Walbrook, Md., testing plant at.56-57 

Webster process, description of.26-27 

Weldon chlorine process, description of. 17-18* 

Whipple, Leyland, work of. 35 

Whitman, E. B., work of. 7,35 

Wollheim, H., process of. 32 

Woodhead, Sims, investigations of. 9 

Woolf process, description of. 27 

Worcester, Mass., acid in sewage of. 17 

Z. 

Zim and Dunbar, experiments of. 19 


















































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